Add Prometheus exporter example (#3168)

.github/dependabot.yml

@@ -100,6 +100,15 @@ updates:
     schedule:
       interval: weekly
       day: sunday
+  - package-ecosystem: gomod
+    directory: /example/prometheus
+    labels:
+      - dependencies
+      - go
+      - Skip Changelog
+    schedule:
+      interval: weekly
+      day: sunday
   - package-ecosystem: gomod
     directory: /example/zipkin
     labels:

example/prometheus/doc.go

@@ -0,0 +1,16 @@
+// 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 main provides a code sample of the Prometheus exporter.
+package main

example/prometheus/go.mod

@@ -0,0 +1,39 @@
+module github.com/open-telemetry/opentelemetry-go/example/prometheus
+
+go 1.18
+
+require (
+	github.com/prometheus/client_golang v1.13.0
+	go.opentelemetry.io/otel v1.9.0
+	go.opentelemetry.io/otel/exporters/prometheus v0.31.0
+	go.opentelemetry.io/otel/metric v0.31.0
+	go.opentelemetry.io/otel/sdk/metric v0.31.0
+)
+
+require (
+	github.com/beorn7/perks v1.0.1 // indirect
+	github.com/cespare/xxhash/v2 v2.1.2 // indirect
+	github.com/go-logr/logr v1.2.3 // indirect
+	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_model v0.2.0 // indirect
+	github.com/prometheus/common v0.37.0 // indirect
+	github.com/prometheus/procfs v0.8.0 // indirect
+	go.opentelemetry.io/otel/sdk v1.8.0 // indirect
+	go.opentelemetry.io/otel/trace v1.9.0 // indirect
+	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a // indirect
+	google.golang.org/protobuf v1.28.1 // indirect
+)
+
+replace go.opentelemetry.io/otel => ../..
+
+replace go.opentelemetry.io/otel/exporters/prometheus => ../../exporters/prometheus
+
+replace go.opentelemetry.io/otel/sdk => ../../sdk
+
+replace go.opentelemetry.io/otel/sdk/metric => ../../sdk/metric
+
+replace go.opentelemetry.io/otel/metric => ../../metric
+
+replace go.opentelemetry.io/otel/trace => ../../trace