From c5b4b9b6b7a1b1122e40e4bd4a27c2090439d428 Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Wed, 28 Sep 2022 20:02:43 +0000
Subject: [PATCH 1/8] refactor prometheus exporter

This change will automatically register the exporter with prometheus, and provides an httpHandler for triggering collect
---
 example/prometheus/go.mod              |   2 +-
 example/prometheus/main.go             |  27 +++---
 example/view/main.go                   |   5 +-
 exporters/prometheus/benchmark_test.go |   8 +-
 exporters/prometheus/confg_test.go     | 109 +++++++++++++++++++++++++
 exporters/prometheus/config.go         |  74 +++++++++++++++++
 exporters/prometheus/exporter.go       |  52 +++++++-----
 exporters/prometheus/exporter_test.go  |   8 +-
 8 files changed, 236 insertions(+), 49 deletions(-)
 create mode 100644 exporters/prometheus/confg_test.go
 create mode 100644 exporters/prometheus/config.go

diff --git a/example/prometheus/go.mod b/example/prometheus/go.mod
index 1672cc93961..1a597fc8d1a 100644
--- a/example/prometheus/go.mod
+++ b/example/prometheus/go.mod
@@ -3,7 +3,6 @@ module go.opentelemetry.io/otel/example/prometheus
 go 1.18
 
 require (
-	github.com/prometheus/client_golang v1.13.0
 	go.opentelemetry.io/otel v1.10.0
 	go.opentelemetry.io/otel/exporters/prometheus v0.32.1
 	go.opentelemetry.io/otel/metric v0.32.1
@@ -17,6 +16,7 @@ require (
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
 	github.com/matttproud/golang_protobuf_extensions v1.0.1 // indirect
+	github.com/prometheus/client_golang v1.13.0 // indirect
 	github.com/prometheus/client_model v0.2.0 // indirect
 	github.com/prometheus/common v0.37.0 // indirect
 	github.com/prometheus/procfs v0.8.0 // indirect
diff --git a/example/prometheus/main.go b/example/prometheus/main.go
index 0517da33c2f..7a93454d58e 100644
--- a/example/prometheus/main.go
+++ b/example/prometheus/main.go
@@ -22,11 +22,8 @@ import (
 	"os"
 	"os/signal"
 
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promhttp"
-
 	"go.opentelemetry.io/otel/attribute"
-	otelprom "go.opentelemetry.io/otel/exporters/prometheus"
+	"go.opentelemetry.io/otel/exporters/prometheus"
 	"go.opentelemetry.io/otel/metric/instrument"
 	"go.opentelemetry.io/otel/sdk/metric"
 )
@@ -37,12 +34,15 @@ func main() {
 	// The exporter embeds a default OpenTelemetry Reader and
 	// implements prometheus.Collector, allowing it to be used as
 	// both a Reader and Collector.
-	exporter := otelprom.New()
+	exporter, err := prometheus.New()
+	if err != nil {
+		log.Fatal(err)
+	}
 	provider := metric.NewMeterProvider(metric.WithReader(exporter))
 	meter := provider.Meter("github.com/open-telemetry/opentelemetry-go/example/prometheus")
 
 	// Start the prometheus HTTP server and pass the exporter Collector to it
-	go serveMetrics(exporter.Collector)
+	go serveMetrics(exporter)
 
 	attrs := []attribute.KeyValue{
 		attribute.Key("A").String("B"),
@@ -77,17 +77,10 @@ func main() {
 	<-ctx.Done()
 }
 
-func serveMetrics(collector prometheus.Collector) {
-	registry := prometheus.NewRegistry()
-	err := registry.Register(collector)
-	if err != nil {
-		fmt.Printf("error registering collector: %v", err)
-		return
-	}
-
-	log.Printf("serving metrics at localhost:2222/metrics")
-	http.Handle("/metrics", promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
-	err = http.ListenAndServe(":2222", nil)
+func serveMetrics(exp *prometheus.Exporter) {
+	log.Printf("serving metrics at localhost:2223/metrics")
+	http.Handle("/metrics", exp)
+	err := http.ListenAndServe(":2223", nil)
 	if err != nil {
 		fmt.Printf("error serving http: %v", err)
 		return
diff --git a/example/view/main.go b/example/view/main.go
index 872e12dde1f..5e7eb8265a6 100644
--- a/example/view/main.go
+++ b/example/view/main.go
@@ -40,7 +40,10 @@ func main() {
 	ctx := context.Background()
 
 	// The exporter embeds a default OpenTelemetry Reader, allowing it to be used in WithReader.
-	exporter := otelprom.New()
+	exporter, err := otelprom.New()
+	if err != nil {
+		log.Fatal(err)
+	}
 
 	// View to customize histogram buckets and rename a single histogram instrument.
 	customBucketsView, err := view.New(
diff --git a/exporters/prometheus/benchmark_test.go b/exporters/prometheus/benchmark_test.go
index dee0814ed75..224626ee781 100644
--- a/exporters/prometheus/benchmark_test.go
+++ b/exporters/prometheus/benchmark_test.go
@@ -27,14 +27,12 @@ import (
 
 func benchmarkCollect(b *testing.B, n int) {
 	ctx := context.Background()
-	exporter := New()
+	registry := prometheus.NewRegistry() // This is the default behavior, this is used to manually gather.
+	exporter, err := New(WithRegistry(registry))
+	require.NoError(b, err)
 	provider := metric.NewMeterProvider(metric.WithReader(exporter))
 	meter := provider.Meter("testmeter")
 
-	registry := prometheus.NewRegistry()
-	err := registry.Register(exporter.Collector)
-	require.NoError(b, err)
-
 	for i := 0; i < n; i++ {
 		counter, err := meter.SyncFloat64().Counter(fmt.Sprintf("foo_%d", i))
 		require.NoError(b, err)
diff --git a/exporters/prometheus/confg_test.go b/exporters/prometheus/confg_test.go
new file mode 100644
index 00000000000..7210ea97a76
--- /dev/null
+++ b/exporters/prometheus/confg_test.go
@@ -0,0 +1,109 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
+
+import (
+	"context"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/metric/metricdata"
+)
+
+type testExporter struct{}
+
+func (e testExporter) Export(_ context.Context, _ metricdata.ResourceMetrics) error {
+	return nil
+}
+
+func (e testExporter) ForceFlush(_ context.Context) error {
+	return nil
+}
+
+func (e testExporter) Shutdown(_ context.Context) error {
+	return nil
+}
+
+func TestNewConfig(t *testing.T) {
+	registry := prometheus.NewRegistry()
+
+	testCases := []struct {
+		name           string
+		options        []Option
+		wantReaderType metric.Reader
+		wantRegisterer prometheus.Registerer
+		wantGatherer   prometheus.Gatherer
+	}{
+		{
+			name:           "Default",
+			options:        nil,
+			wantReaderType: metric.NewManualReader(),
+			wantRegisterer: prometheus.DefaultRegisterer,
+			wantGatherer:   prometheus.DefaultGatherer,
+		},
+		{
+			name: "WithReader",
+			options: []Option{
+				WithReader(metric.NewPeriodicReader(testExporter{})),
+			},
+			wantReaderType: metric.NewPeriodicReader(testExporter{}),
+			wantRegisterer: prometheus.DefaultRegisterer,
+			wantGatherer:   prometheus.DefaultGatherer,
+		},
+		{
+			name: "WithRegistry",
+			options: []Option{
+				WithRegistry(registry),
+			},
+			wantReaderType: metric.NewManualReader(),
+			wantRegisterer: registry,
+			wantGatherer:   registry,
+		},
+		{
+			name: "Multiple Options",
+			options: []Option{
+				WithReader(metric.NewPeriodicReader(testExporter{})),
+				WithRegistry(registry),
+			},
+			wantReaderType: metric.NewPeriodicReader(testExporter{}),
+			wantRegisterer: registry,
+			wantGatherer:   registry,
+		},
+		{
+			name: "nil options do nothing",
+			options: []Option{
+				WithReader(nil),
+				WithRegistry(nil),
+			},
+			wantReaderType: metric.NewManualReader(),
+			wantRegisterer: prometheus.DefaultRegisterer,
+			wantGatherer:   prometheus.DefaultGatherer,
+		},
+	}
+	for _, tt := range testCases {
+		t.Run(tt.name, func(t *testing.T) {
+			cfg := newConfig(tt.options...)
+
+			// If no reader is provided you should get a new ManualReader.
+			assert.IsType(t, tt.wantReaderType, cfg.reader)
+
+			// If no Registry is provided you should get the DefaultRegisterer and DefaultGatherer.
+			assert.Equal(t, tt.wantRegisterer, cfg.registerer)
+			assert.Equal(t, tt.wantGatherer, cfg.gatherer)
+		})
+	}
+}
diff --git a/exporters/prometheus/config.go b/exporters/prometheus/config.go
new file mode 100644
index 00000000000..43bea2f9228
--- /dev/null
+++ b/exporters/prometheus/config.go
@@ -0,0 +1,74 @@
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+
+	"go.opentelemetry.io/otel/sdk/metric"
+) // config is added here to allow for options expansion in the future.
+type config struct {
+	reader metric.Reader
+
+	registry   *prometheus.Registry
+	registerer prometheus.Registerer
+	gatherer   prometheus.Gatherer
+}
+
+func newConfig(opts ...Option) config {
+	cfg := config{}
+	for _, opt := range opts {
+		cfg = opt.apply(cfg)
+	}
+
+	if cfg.reader == nil {
+		cfg.reader = metric.NewManualReader()
+	}
+
+	if cfg.registry != nil {
+		cfg.registerer = cfg.registry
+		cfg.gatherer = cfg.registry
+	} else {
+		cfg.registerer = prometheus.DefaultRegisterer
+		cfg.gatherer = prometheus.DefaultGatherer
+	}
+
+	return cfg
+}
+
+// Option may be used in the future to apply options to a Prometheus Exporter config.
+type Option interface {
+	apply(config) config
+}
+
+type optionFunc func(config) config
+
+func (fn optionFunc) apply(cfg config) config {
+	return fn(cfg)
+}
+
+func WithReader(rdr metric.Reader) Option {
+	return optionFunc(func(cfg config) config {
+		cfg.reader = rdr
+		return cfg
+	})
+}
+
+func WithRegistry(reg *prometheus.Registry) Option {
+	return optionFunc(func(cfg config) config {
+		cfg.registry = reg
+		return cfg
+	})
+}
diff --git a/exporters/prometheus/exporter.go b/exporters/prometheus/exporter.go
index 3fe848f1e03..7f48603ad88 100644
--- a/exporters/prometheus/exporter.go
+++ b/exporters/prometheus/exporter.go
@@ -16,12 +16,15 @@ package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
 
 import (
 	"context"
+	"fmt"
+	"net/http"
 	"sort"
 	"strings"
 	"unicode"
 	"unicode/utf8"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
@@ -34,38 +37,47 @@ import (
 type Exporter struct {
 	metric.Reader
 	Collector prometheus.Collector
+
+	handler http.Handler
 }
 
+var _ metric.Reader = &Exporter{}
+
 // collector is used to implement prometheus.Collector.
 type collector struct {
-	metric.Reader
+	reader metric.Reader
 }
 
-// config is added here to allow for options expansion in the future.
-type config struct{}
+// New returns a Prometheus Exporter.
+func New(opts ...Option) (*Exporter, error) {
+	cfg := newConfig(opts...)
 
-// Option may be used in the future to apply options to a Prometheus Exporter config.
-type Option interface {
-	apply(config) config
-}
+	handler := promhttp.HandlerFor(cfg.gatherer, promhttp.HandlerOpts{})
+	collector := &collector{
+		reader: cfg.reader,
+	}
 
-// New returns a Prometheus Exporter.
-func New(_ ...Option) Exporter {
-	// this assumes that the default temporality selector will always return cumulative.
-	// we only support cumulative temporality, so building our own reader enforces this.
-	reader := metric.NewManualReader()
-	e := Exporter{
-		Reader: reader,
-		Collector: &collector{
-			Reader: reader,
-		},
+	if err := cfg.registerer.Register(collector); err != nil {
+		return nil, fmt.Errorf("cannot register the collector: %w", err)
 	}
-	return e
+
+	e := &Exporter{
+		Reader:    cfg.reader,
+		Collector: collector,
+
+		handler: handler,
+	}
+
+	return e, nil
+}
+
+func (e *Exporter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	e.handler.ServeHTTP(w, r)
 }
 
 // Describe implements prometheus.Collector.
 func (c *collector) Describe(ch chan<- *prometheus.Desc) {
-	metrics, err := c.Reader.Collect(context.TODO())
+	metrics, err := c.reader.Collect(context.TODO())
 	if err != nil {
 		otel.Handle(err)
 	}
@@ -76,7 +88,7 @@ func (c *collector) Describe(ch chan<- *prometheus.Desc) {
 
 // Collect implements prometheus.Collector.
 func (c *collector) Collect(ch chan<- prometheus.Metric) {
-	metrics, err := c.Reader.Collect(context.TODO())
+	metrics, err := c.reader.Collect(context.TODO())
 	if err != nil {
 		otel.Handle(err)
 	}
diff --git a/exporters/prometheus/exporter_test.go b/exporters/prometheus/exporter_test.go
index 6d46c3be0db..8de3afd133a 100644
--- a/exporters/prometheus/exporter_test.go
+++ b/exporters/prometheus/exporter_test.go
@@ -135,15 +135,13 @@ func TestPrometheusExporter(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			ctx := context.Background()
+			registry := prometheus.NewRegistry() // This is the default behavior, this is used to manually gather.
 
-			exporter := New()
+			exporter, err := New(WithRegistry(registry))
+			require.NoError(t, err)
 			provider := metric.NewMeterProvider(metric.WithReader(exporter))
 			meter := provider.Meter("testmeter")
 
-			registry := prometheus.NewRegistry()
-			err := registry.Register(exporter.Collector)
-			require.NoError(t, err)
-
 			tc.recordMetrics(ctx, meter)
 
 			file, err := os.Open(tc.expectedFile)

From 5b92528b490f3bb62091f69de103bf9db69cad51 Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Thu, 29 Sep 2022 16:03:01 +0000
Subject: [PATCH 2/8] Fix Lint Remote WithRegistry Adds WithRegisterer and
 WithGatherer

---
 exporters/prometheus/config.go | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/exporters/prometheus/config.go b/exporters/prometheus/config.go
index 43bea2f9228..37cae9a742d 100644
--- a/exporters/prometheus/config.go
+++ b/exporters/prometheus/config.go
@@ -22,7 +22,6 @@ import (
 type config struct {
 	reader metric.Reader
 
-	registry   *prometheus.Registry
 	registerer prometheus.Registerer
 	gatherer   prometheus.Gatherer
 }
@@ -37,13 +36,12 @@ func newConfig(opts ...Option) config {
 		cfg.reader = metric.NewManualReader()
 	}
 
-	if cfg.registry != nil {
-		cfg.registerer = cfg.registry
-		cfg.gatherer = cfg.registry
-	} else {
-		cfg.registerer = prometheus.DefaultRegisterer
+	if cfg.gatherer == nil {
 		cfg.gatherer = prometheus.DefaultGatherer
 	}
+	if cfg.registerer == nil {
+		cfg.registerer = prometheus.DefaultRegisterer
+	}
 
 	return cfg
 }
@@ -59,6 +57,8 @@ func (fn optionFunc) apply(cfg config) config {
 	return fn(cfg)
 }
 
+// WithReader controls where the Exporter reader Collects() from.  If no reader
+// is passed a ManualReader will be used.
 func WithReader(rdr metric.Reader) Option {
 	return optionFunc(func(cfg config) config {
 		cfg.reader = rdr
@@ -66,9 +66,22 @@ func WithReader(rdr metric.Reader) Option {
 	})
 }
 
-func WithRegistry(reg *prometheus.Registry) Option {
+// WithRegisterer configures which prometheus Registerer the Exporter will
+// register with.  If no registerer is used the prometheus DefaultRegisterer is
+// used.
+func WithRegisterer(reg prometheus.Registerer) Option {
+	return optionFunc(func(cfg config) config {
+		cfg.registerer = reg
+		return cfg
+	})
+}
+
+// WithRegisterer configures which prometheus Gatherer the Exporter will
+// Gather from.  If no gatherer is used the prometheus DefaultGatherer is
+// used.
+func WithGatherer(gatherer prometheus.Gatherer) Option {
 	return optionFunc(func(cfg config) config {
-		cfg.registry = reg
+		cfg.gatherer = gatherer
 		return cfg
 	})
 }

From 96abfede3d5594ebf50a4c16cfef4a78b0e3440a Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Thu, 29 Sep 2022 16:51:14 +0000
Subject: [PATCH 3/8] Fix Tests

---
 exporters/prometheus/benchmark_test.go |  4 ++--
 exporters/prometheus/confg_test.go     | 21 ++++++++++++++++-----
 exporters/prometheus/exporter_test.go  |  4 ++--
 3 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/exporters/prometheus/benchmark_test.go b/exporters/prometheus/benchmark_test.go
index 224626ee781..be85665b951 100644
--- a/exporters/prometheus/benchmark_test.go
+++ b/exporters/prometheus/benchmark_test.go
@@ -27,8 +27,8 @@ import (
 
 func benchmarkCollect(b *testing.B, n int) {
 	ctx := context.Background()
-	registry := prometheus.NewRegistry() // This is the default behavior, this is used to manually gather.
-	exporter, err := New(WithRegistry(registry))
+	registry := prometheus.NewRegistry()
+	exporter, err := New(WithGatherer(registry), WithRegisterer(registry))
 	require.NoError(b, err)
 	provider := metric.NewMeterProvider(metric.WithReader(exporter))
 	meter := provider.Meter("testmeter")
diff --git a/exporters/prometheus/confg_test.go b/exporters/prometheus/confg_test.go
index 7210ea97a76..1cb96f5172f 100644
--- a/exporters/prometheus/confg_test.go
+++ b/exporters/prometheus/confg_test.go
@@ -65,19 +65,29 @@ func TestNewConfig(t *testing.T) {
 			wantGatherer:   prometheus.DefaultGatherer,
 		},
 		{
-			name: "WithRegistry",
+			name: "WithGatherer",
 			options: []Option{
-				WithRegistry(registry),
+				WithGatherer(registry),
 			},
 			wantReaderType: metric.NewManualReader(),
-			wantRegisterer: registry,
+			wantRegisterer: prometheus.DefaultRegisterer,
 			wantGatherer:   registry,
 		},
+		{
+			name: "WithRegisterer",
+			options: []Option{
+				WithRegisterer(registry),
+			},
+			wantReaderType: metric.NewManualReader(),
+			wantRegisterer: registry,
+			wantGatherer:   prometheus.DefaultGatherer,
+		},
 		{
 			name: "Multiple Options",
 			options: []Option{
 				WithReader(metric.NewPeriodicReader(testExporter{})),
-				WithRegistry(registry),
+				WithGatherer(registry),
+				WithRegisterer(registry),
 			},
 			wantReaderType: metric.NewPeriodicReader(testExporter{}),
 			wantRegisterer: registry,
@@ -87,7 +97,8 @@ func TestNewConfig(t *testing.T) {
 			name: "nil options do nothing",
 			options: []Option{
 				WithReader(nil),
-				WithRegistry(nil),
+				WithGatherer(nil),
+				WithRegisterer(nil),
 			},
 			wantReaderType: metric.NewManualReader(),
 			wantRegisterer: prometheus.DefaultRegisterer,
diff --git a/exporters/prometheus/exporter_test.go b/exporters/prometheus/exporter_test.go
index 8de3afd133a..38452192162 100644
--- a/exporters/prometheus/exporter_test.go
+++ b/exporters/prometheus/exporter_test.go
@@ -135,9 +135,9 @@ func TestPrometheusExporter(t *testing.T) {
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
 			ctx := context.Background()
-			registry := prometheus.NewRegistry() // This is the default behavior, this is used to manually gather.
+			registry := prometheus.NewRegistry()
 
-			exporter, err := New(WithRegistry(registry))
+			exporter, err := New(WithGatherer(registry), WithRegisterer(registry))
 			require.NoError(t, err)
 			provider := metric.NewMeterProvider(metric.WithReader(exporter))
 			meter := provider.Meter("testmeter")

From 406817dab0db3d17b0f6705751fae9201b249ba5 Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Thu, 29 Sep 2022 16:52:30 +0000
Subject: [PATCH 4/8] Fix lint

---
 exporters/prometheus/confg_test.go | 1 +
 1 file changed, 1 insertion(+)

diff --git a/exporters/prometheus/confg_test.go b/exporters/prometheus/confg_test.go
index 1cb96f5172f..0a409e07cd7 100644
--- a/exporters/prometheus/confg_test.go
+++ b/exporters/prometheus/confg_test.go
@@ -20,6 +20,7 @@ import (
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
+
 	"go.opentelemetry.io/otel/sdk/metric"
 	"go.opentelemetry.io/otel/sdk/metric/metricdata"
 )

From cbb800151be1cc5db2414315bf1785019bfb0afc Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Thu, 29 Sep 2022 20:44:58 +0000
Subject: [PATCH 5/8] Removed reader config can be added after (#3244)

---
 exporters/prometheus/confg_test.go | 39 +-----------------------------
 exporters/prometheus/config.go     | 17 -------------
 exporters/prometheus/exporter.go   |  7 ++++--
 3 files changed, 6 insertions(+), 57 deletions(-)

diff --git a/exporters/prometheus/confg_test.go b/exporters/prometheus/confg_test.go
index 0a409e07cd7..fda6641bcaa 100644
--- a/exporters/prometheus/confg_test.go
+++ b/exporters/prometheus/confg_test.go
@@ -15,62 +15,33 @@
 package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
 
 import (
-	"context"
 	"testing"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
-
-	"go.opentelemetry.io/otel/sdk/metric"
-	"go.opentelemetry.io/otel/sdk/metric/metricdata"
 )
 
-type testExporter struct{}
-
-func (e testExporter) Export(_ context.Context, _ metricdata.ResourceMetrics) error {
-	return nil
-}
-
-func (e testExporter) ForceFlush(_ context.Context) error {
-	return nil
-}
-
-func (e testExporter) Shutdown(_ context.Context) error {
-	return nil
-}
-
 func TestNewConfig(t *testing.T) {
 	registry := prometheus.NewRegistry()
 
 	testCases := []struct {
 		name           string
 		options        []Option
-		wantReaderType metric.Reader
 		wantRegisterer prometheus.Registerer
 		wantGatherer   prometheus.Gatherer
 	}{
 		{
 			name:           "Default",
 			options:        nil,
-			wantReaderType: metric.NewManualReader(),
-			wantRegisterer: prometheus.DefaultRegisterer,
-			wantGatherer:   prometheus.DefaultGatherer,
-		},
-		{
-			name: "WithReader",
-			options: []Option{
-				WithReader(metric.NewPeriodicReader(testExporter{})),
-			},
-			wantReaderType: metric.NewPeriodicReader(testExporter{}),
 			wantRegisterer: prometheus.DefaultRegisterer,
 			wantGatherer:   prometheus.DefaultGatherer,
 		},
+
 		{
 			name: "WithGatherer",
 			options: []Option{
 				WithGatherer(registry),
 			},
-			wantReaderType: metric.NewManualReader(),
 			wantRegisterer: prometheus.DefaultRegisterer,
 			wantGatherer:   registry,
 		},
@@ -79,29 +50,24 @@ func TestNewConfig(t *testing.T) {
 			options: []Option{
 				WithRegisterer(registry),
 			},
-			wantReaderType: metric.NewManualReader(),
 			wantRegisterer: registry,
 			wantGatherer:   prometheus.DefaultGatherer,
 		},
 		{
 			name: "Multiple Options",
 			options: []Option{
-				WithReader(metric.NewPeriodicReader(testExporter{})),
 				WithGatherer(registry),
 				WithRegisterer(registry),
 			},
-			wantReaderType: metric.NewPeriodicReader(testExporter{}),
 			wantRegisterer: registry,
 			wantGatherer:   registry,
 		},
 		{
 			name: "nil options do nothing",
 			options: []Option{
-				WithReader(nil),
 				WithGatherer(nil),
 				WithRegisterer(nil),
 			},
-			wantReaderType: metric.NewManualReader(),
 			wantRegisterer: prometheus.DefaultRegisterer,
 			wantGatherer:   prometheus.DefaultGatherer,
 		},
@@ -110,9 +76,6 @@ func TestNewConfig(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			cfg := newConfig(tt.options...)
 
-			// If no reader is provided you should get a new ManualReader.
-			assert.IsType(t, tt.wantReaderType, cfg.reader)
-
 			// If no Registry is provided you should get the DefaultRegisterer and DefaultGatherer.
 			assert.Equal(t, tt.wantRegisterer, cfg.registerer)
 			assert.Equal(t, tt.wantGatherer, cfg.gatherer)
diff --git a/exporters/prometheus/config.go b/exporters/prometheus/config.go
index 37cae9a742d..65e85d675d6 100644
--- a/exporters/prometheus/config.go
+++ b/exporters/prometheus/config.go
@@ -16,12 +16,8 @@ package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
 
 import (
 	"github.com/prometheus/client_golang/prometheus"
-
-	"go.opentelemetry.io/otel/sdk/metric"
 ) // config is added here to allow for options expansion in the future.
 type config struct {
-	reader metric.Reader
-
 	registerer prometheus.Registerer
 	gatherer   prometheus.Gatherer
 }
@@ -32,10 +28,6 @@ func newConfig(opts ...Option) config {
 		cfg = opt.apply(cfg)
 	}
 
-	if cfg.reader == nil {
-		cfg.reader = metric.NewManualReader()
-	}
-
 	if cfg.gatherer == nil {
 		cfg.gatherer = prometheus.DefaultGatherer
 	}
@@ -57,15 +49,6 @@ func (fn optionFunc) apply(cfg config) config {
 	return fn(cfg)
 }
 
-// WithReader controls where the Exporter reader Collects() from.  If no reader
-// is passed a ManualReader will be used.
-func WithReader(rdr metric.Reader) Option {
-	return optionFunc(func(cfg config) config {
-		cfg.reader = rdr
-		return cfg
-	})
-}
-
 // WithRegisterer configures which prometheus Registerer the Exporter will
 // register with.  If no registerer is used the prometheus DefaultRegisterer is
 // used.
diff --git a/exporters/prometheus/exporter.go b/exporters/prometheus/exporter.go
index 7f48603ad88..47467468d44 100644
--- a/exporters/prometheus/exporter.go
+++ b/exporters/prometheus/exporter.go
@@ -52,9 +52,12 @@ type collector struct {
 func New(opts ...Option) (*Exporter, error) {
 	cfg := newConfig(opts...)
 
+	// TODO (#????): Enable some way to configure the reader, but not change temporality.
+	reader := metric.NewManualReader()
+
 	handler := promhttp.HandlerFor(cfg.gatherer, promhttp.HandlerOpts{})
 	collector := &collector{
-		reader: cfg.reader,
+		reader: reader,
 	}
 
 	if err := cfg.registerer.Register(collector); err != nil {
@@ -62,7 +65,7 @@ func New(opts ...Option) (*Exporter, error) {
 	}
 
 	e := &Exporter{
-		Reader:    cfg.reader,
+		Reader:    reader,
 		Collector: collector,
 
 		handler: handler,

From c00a0fabf92f98ae10cae9cff3459ad613cc0de7 Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Thu, 6 Oct 2022 18:25:23 +0000
Subject: [PATCH 6/8] Add changelog

---
 CHANGELOG.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cb99127ad4f..63675c8caf4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Added
 
 - Added an example of using metric views to customize instruments. (#3177)
+- Prometheus exporter will register with a prometheus registerer on creation, there are options to control this (#3239)
 
 ### Changed
 

From 0e3bbb920faf753fe34705524853bb301fb6d63b Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Fri, 7 Oct 2022 15:04:11 +0000
Subject: [PATCH 7/8] Update with PR feedback

---
 CHANGELOG.md                           |  3 ++-
 example/prometheus/go.mod              |  2 +-
 example/prometheus/main.go             |  8 +++++---
 example/view/main.go                   | 16 ++++------------
 exporters/prometheus/benchmark_test.go |  2 +-
 exporters/prometheus/confg_test.go     | 23 -----------------------
 exporters/prometheus/config.go         | 18 +++---------------
 exporters/prometheus/exporter.go       | 19 ++++---------------
 exporters/prometheus/exporter_test.go  |  2 +-
 9 files changed, 21 insertions(+), 72 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 63675c8caf4..e324643330c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,7 +11,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Added
 
 - Added an example of using metric views to customize instruments. (#3177)
-- Prometheus exporter will register with a prometheus registerer on creation, there are options to control this (#3239)
+- Prometheus exporter will register with a prometheus registerer on creation, there are options to control this. (#3239)
 
 ### Changed
 
@@ -20,6 +20,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Upgrade `golang.org/x/sys/unix` from `v0.0.0-20210423185535-09eb48e85fd7` to `v0.0.0-20220919091848-fb04ddd9f9c8`.
   This addresses [GO-2022-0493](https://pkg.go.dev/vuln/GO-2022-0493). (#3235)
 - Update histogram default bounds to match the requirements of the latest specification. (#3222)
+- Prometheus New() function now also returns an error. (#3239)
 
 ## [0.32.1] Metric SDK (Alpha) - 2022-09-22
 
diff --git a/example/prometheus/go.mod b/example/prometheus/go.mod
index 1a597fc8d1a..1672cc93961 100644
--- a/example/prometheus/go.mod
+++ b/example/prometheus/go.mod
@@ -3,6 +3,7 @@ module go.opentelemetry.io/otel/example/prometheus
 go 1.18
 
 require (
+	github.com/prometheus/client_golang v1.13.0
 	go.opentelemetry.io/otel v1.10.0
 	go.opentelemetry.io/otel/exporters/prometheus v0.32.1
 	go.opentelemetry.io/otel/metric v0.32.1
@@ -16,7 +17,6 @@ require (
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/golang/protobuf v1.5.2 // indirect
 	github.com/matttproud/golang_protobuf_extensions v1.0.1 // indirect
-	github.com/prometheus/client_golang v1.13.0 // indirect
 	github.com/prometheus/client_model v0.2.0 // indirect
 	github.com/prometheus/common v0.37.0 // indirect
 	github.com/prometheus/procfs v0.8.0 // indirect
diff --git a/example/prometheus/main.go b/example/prometheus/main.go
index 7a93454d58e..e1d87693c22 100644
--- a/example/prometheus/main.go
+++ b/example/prometheus/main.go
@@ -22,6 +22,8 @@ import (
 	"os"
 	"os/signal"
 
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+
 	"go.opentelemetry.io/otel/attribute"
 	"go.opentelemetry.io/otel/exporters/prometheus"
 	"go.opentelemetry.io/otel/metric/instrument"
@@ -42,7 +44,7 @@ func main() {
 	meter := provider.Meter("github.com/open-telemetry/opentelemetry-go/example/prometheus")
 
 	// Start the prometheus HTTP server and pass the exporter Collector to it
-	go serveMetrics(exporter)
+	go serveMetrics()
 
 	attrs := []attribute.KeyValue{
 		attribute.Key("A").String("B"),
@@ -77,9 +79,9 @@ func main() {
 	<-ctx.Done()
 }
 
-func serveMetrics(exp *prometheus.Exporter) {
+func serveMetrics() {
 	log.Printf("serving metrics at localhost:2223/metrics")
-	http.Handle("/metrics", exp)
+	http.Handle("/metrics", promhttp.Handler())
 	err := http.ListenAndServe(":2223", nil)
 	if err != nil {
 		fmt.Printf("error serving http: %v", err)
diff --git a/example/view/main.go b/example/view/main.go
index 5e7eb8265a6..c8f1b246590 100644
--- a/example/view/main.go
+++ b/example/view/main.go
@@ -22,7 +22,6 @@ import (
 	"os"
 	"os/signal"
 
-	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 
 	"go.opentelemetry.io/otel/attribute"
@@ -71,7 +70,7 @@ func main() {
 	meter := provider.Meter(meterName)
 
 	// Start the prometheus HTTP server and pass the exporter Collector to it
-	go serveMetrics(exporter.Collector)
+	go serveMetrics()
 
 	attrs := []attribute.KeyValue{
 		attribute.Key("A").String("B"),
@@ -97,17 +96,10 @@ func main() {
 	<-ctx.Done()
 }
 
-func serveMetrics(collector prometheus.Collector) {
-	registry := prometheus.NewRegistry()
-	err := registry.Register(collector)
-	if err != nil {
-		fmt.Printf("error registering collector: %v", err)
-		return
-	}
-
+func serveMetrics() {
 	log.Printf("serving metrics at localhost:2222/metrics")
-	http.Handle("/metrics", promhttp.HandlerFor(registry, promhttp.HandlerOpts{}))
-	err = http.ListenAndServe(":2222", nil)
+	http.Handle("/metrics", promhttp.Handler())
+	err := http.ListenAndServe(":2222", nil)
 	if err != nil {
 		fmt.Printf("error serving http: %v", err)
 		return
diff --git a/exporters/prometheus/benchmark_test.go b/exporters/prometheus/benchmark_test.go
index be85665b951..6c6d67cae76 100644
--- a/exporters/prometheus/benchmark_test.go
+++ b/exporters/prometheus/benchmark_test.go
@@ -28,7 +28,7 @@ import (
 func benchmarkCollect(b *testing.B, n int) {
 	ctx := context.Background()
 	registry := prometheus.NewRegistry()
-	exporter, err := New(WithGatherer(registry), WithRegisterer(registry))
+	exporter, err := New(WithRegisterer(registry))
 	require.NoError(b, err)
 	provider := metric.NewMeterProvider(metric.WithReader(exporter))
 	meter := provider.Meter("testmeter")
diff --git a/exporters/prometheus/confg_test.go b/exporters/prometheus/confg_test.go
index fda6641bcaa..29c63a97572 100644
--- a/exporters/prometheus/confg_test.go
+++ b/exporters/prometheus/confg_test.go
@@ -28,48 +28,26 @@ func TestNewConfig(t *testing.T) {
 		name           string
 		options        []Option
 		wantRegisterer prometheus.Registerer
-		wantGatherer   prometheus.Gatherer
 	}{
 		{
 			name:           "Default",
 			options:        nil,
 			wantRegisterer: prometheus.DefaultRegisterer,
-			wantGatherer:   prometheus.DefaultGatherer,
 		},
 
-		{
-			name: "WithGatherer",
-			options: []Option{
-				WithGatherer(registry),
-			},
-			wantRegisterer: prometheus.DefaultRegisterer,
-			wantGatherer:   registry,
-		},
 		{
 			name: "WithRegisterer",
 			options: []Option{
 				WithRegisterer(registry),
 			},
 			wantRegisterer: registry,
-			wantGatherer:   prometheus.DefaultGatherer,
-		},
-		{
-			name: "Multiple Options",
-			options: []Option{
-				WithGatherer(registry),
-				WithRegisterer(registry),
-			},
-			wantRegisterer: registry,
-			wantGatherer:   registry,
 		},
 		{
 			name: "nil options do nothing",
 			options: []Option{
-				WithGatherer(nil),
 				WithRegisterer(nil),
 			},
 			wantRegisterer: prometheus.DefaultRegisterer,
-			wantGatherer:   prometheus.DefaultGatherer,
 		},
 	}
 	for _, tt := range testCases {
@@ -78,7 +56,6 @@ func TestNewConfig(t *testing.T) {
 
 			// If no Registry is provided you should get the DefaultRegisterer and DefaultGatherer.
 			assert.Equal(t, tt.wantRegisterer, cfg.registerer)
-			assert.Equal(t, tt.wantGatherer, cfg.gatherer)
 		})
 	}
 }
diff --git a/exporters/prometheus/config.go b/exporters/prometheus/config.go
index 65e85d675d6..faf09f9a188 100644
--- a/exporters/prometheus/config.go
+++ b/exporters/prometheus/config.go
@@ -16,10 +16,11 @@ package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
 
 import (
 	"github.com/prometheus/client_golang/prometheus"
-) // config is added here to allow for options expansion in the future.
+)
+
+// config is added here to allow for options expansion in the future.
 type config struct {
 	registerer prometheus.Registerer
-	gatherer   prometheus.Gatherer
 }
 
 func newConfig(opts ...Option) config {
@@ -28,9 +29,6 @@ func newConfig(opts ...Option) config {
 		cfg = opt.apply(cfg)
 	}
 
-	if cfg.gatherer == nil {
-		cfg.gatherer = prometheus.DefaultGatherer
-	}
 	if cfg.registerer == nil {
 		cfg.registerer = prometheus.DefaultRegisterer
 	}
@@ -58,13 +56,3 @@ func WithRegisterer(reg prometheus.Registerer) Option {
 		return cfg
 	})
 }
-
-// WithRegisterer configures which prometheus Gatherer the Exporter will
-// Gather from.  If no gatherer is used the prometheus DefaultGatherer is
-// used.
-func WithGatherer(gatherer prometheus.Gatherer) Option {
-	return optionFunc(func(cfg config) config {
-		cfg.gatherer = gatherer
-		return cfg
-	})
-}
diff --git a/exporters/prometheus/exporter.go b/exporters/prometheus/exporter.go
index 47467468d44..6f8c5ba7b21 100644
--- a/exporters/prometheus/exporter.go
+++ b/exporters/prometheus/exporter.go
@@ -17,14 +17,12 @@ package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"
 import (
 	"context"
 	"fmt"
-	"net/http"
 	"sort"
 	"strings"
 	"unicode"
 	"unicode/utf8"
 
 	"github.com/prometheus/client_golang/prometheus"
-	"github.com/prometheus/client_golang/prometheus/promhttp"
 
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/attribute"
@@ -36,9 +34,6 @@ import (
 // interface for easy instantiation with a MeterProvider.
 type Exporter struct {
 	metric.Reader
-	Collector prometheus.Collector
-
-	handler http.Handler
 }
 
 var _ metric.Reader = &Exporter{}
@@ -52,10 +47,11 @@ type collector struct {
 func New(opts ...Option) (*Exporter, error) {
 	cfg := newConfig(opts...)
 
-	// TODO (#????): Enable some way to configure the reader, but not change temporality.
+	// this assumes that the default temporality selector will always return cumulative.
+	// we only support cumulative temporality, so building our own reader enforces this.
+	// TODO (#3244): Enable some way to configure the reader, but not change temporality.
 	reader := metric.NewManualReader()
 
-	handler := promhttp.HandlerFor(cfg.gatherer, promhttp.HandlerOpts{})
 	collector := &collector{
 		reader: reader,
 	}
@@ -65,19 +61,12 @@ func New(opts ...Option) (*Exporter, error) {
 	}
 
 	e := &Exporter{
-		Reader:    reader,
-		Collector: collector,
-
-		handler: handler,
+		Reader: reader,
 	}
 
 	return e, nil
 }
 
-func (e *Exporter) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	e.handler.ServeHTTP(w, r)
-}
-
 // Describe implements prometheus.Collector.
 func (c *collector) Describe(ch chan<- *prometheus.Desc) {
 	metrics, err := c.reader.Collect(context.TODO())
diff --git a/exporters/prometheus/exporter_test.go b/exporters/prometheus/exporter_test.go
index 38a7cfd245a..5a1a65ce58d 100644
--- a/exporters/prometheus/exporter_test.go
+++ b/exporters/prometheus/exporter_test.go
@@ -139,7 +139,7 @@ func TestPrometheusExporter(t *testing.T) {
 			ctx := context.Background()
 			registry := prometheus.NewRegistry()
 
-			exporter, err := New(WithGatherer(registry), WithRegisterer(registry))
+			exporter, err := New(WithRegisterer(registry))
 			require.NoError(t, err)
 
 			customBucketsView, err := view.New(

From 0c412fc3183e520117c9d6ee5970ab42c51b2575 Mon Sep 17 00:00:00 2001
From: Aaron Clawson <3766680+MadVikingGod@users.noreply.github.com>
Date: Thu, 13 Oct 2022 14:40:17 +0000
Subject: [PATCH 8/8] Updated comments

---
 exporters/prometheus/confg_test.go | 1 -
 exporters/prometheus/config.go     | 5 +++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/exporters/prometheus/confg_test.go b/exporters/prometheus/confg_test.go
index 29c63a97572..91893d40f3a 100644
--- a/exporters/prometheus/confg_test.go
+++ b/exporters/prometheus/confg_test.go
@@ -54,7 +54,6 @@ func TestNewConfig(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			cfg := newConfig(tt.options...)
 
-			// If no Registry is provided you should get the DefaultRegisterer and DefaultGatherer.
 			assert.Equal(t, tt.wantRegisterer, cfg.registerer)
 		})
 	}
diff --git a/exporters/prometheus/config.go b/exporters/prometheus/config.go
index faf09f9a188..6ee84732556 100644
--- a/exporters/prometheus/config.go
+++ b/exporters/prometheus/config.go
@@ -18,11 +18,12 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-// config is added here to allow for options expansion in the future.
+// config contains options for the exporter.
 type config struct {
 	registerer prometheus.Registerer
 }
 
+// newConfig creates a validated config configured with options.
 func newConfig(opts ...Option) config {
 	cfg := config{}
 	for _, opt := range opts {
@@ -36,7 +37,7 @@ func newConfig(opts ...Option) config {
 	return cfg
 }
 
-// Option may be used in the future to apply options to a Prometheus Exporter config.
+// Option sets exporter option values.
 type Option interface {
 	apply(config) config
 }