Warn instead of failing when renaming metrics using metric_relabel_co…

…nfigs (open-telemetry#25888)

**Description:**

Fixes
open-telemetry#5001

When that issue was filed, we dropped unknown-typed metrics. Now that we
convert them to gauges, users may want to use metric_relabel_configs to
rename metrics, even if they lose metadata. In the future, we have other
enhancements planned to better-support unknown-typed metrics.

Change the error to a warning. It is still likely not a best practice to
use relabel configs for this purpose, but for users copy-pasting
prometheus configs, it is best not to fail if we can help it.

.chloggen/allow_prom_renaming.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: receiver/prometheus
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Warn instead of failing when users rename using metric_relabel_configs in the prometheus receiver
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [5001]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

receiver/prometheusreceiver/config.go

@@ -18,6 +18,7 @@ import (
 	"github.com/prometheus/prometheus/discovery/kubernetes"
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/confmap"
+	"go.uber.org/zap"
 	"gopkg.in/yaml.v2"
 )
 
@@ -142,13 +143,6 @@ func (cfg *Config) validatePromConfig(promConfig *promconfig.Config) error {
 	}
 
 	for _, sc := range cfg.PrometheusConfig.ScrapeConfigs {
-		for _, rc := range sc.MetricRelabelConfigs {
-			if rc.TargetLabel == "__name__" {
-				// TODO(#2297): Remove validation after renaming is fixed
-				return fmt.Errorf("error validating scrapeconfig for job %v: %w", sc.JobName, errRenamingDisallowed)
-			}
-		}
-
 		if sc.HTTPClientConfig.Authorization != nil {
 			if err := checkFile(sc.HTTPClientConfig.Authorization.CredentialsFile); err != nil {
 				return fmt.Errorf("error checking authorization credentials file %q: %w", sc.HTTPClientConfig.Authorization.CredentialsFile, err)
@@ -241,3 +235,13 @@ func (cfg *Config) Unmarshal(componentParser *confmap.Conf) error {
 
 	return nil
 }
+
+func configWarnings(logger *zap.Logger, cfg *Config) {
+	for _, sc := range cfg.PrometheusConfig.ScrapeConfigs {
+		for _, rc := range sc.MetricRelabelConfigs {
+			if rc.TargetLabel == "__name__" {
+				logger.Warn("metric renaming using metric_relabel_configs will result in unknown-typed metrics without a unit or description", zap.String("job", sc.JobName))
+			}
+		}
+	}
+}

receiver/prometheusreceiver/config_test.go

@@ -4,6 +4,7 @@
 package prometheusreceiver
 
 import (
+	"context"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -15,6 +16,9 @@ import (
 	"github.com/stretchr/testify/require"
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/confmap/confmaptest"
+	"go.opentelemetry.io/collector/receiver/receivertest"
+	"go.uber.org/zap"
+	"go.uber.org/zap/zaptest/observer"
 
 	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/prometheusreceiver/internal/metadata"
 )
@@ -120,18 +124,25 @@ func TestLoadConfigFailsOnUnknownPrometheusSection(t *testing.T) {
 	require.Error(t, component.UnmarshalConfig(sub, cfg))
 }
 
-// Renaming is not allowed
-func TestLoadConfigFailsOnRenameDisallowed(t *testing.T) {
-	cm, err := confmaptest.LoadConf(filepath.Join("testdata", "invalid-config-prometheus-relabel.yaml"))
+// Renaming emits a warning
+func TestConfigWarningsOnRenameDisallowed(t *testing.T) {
+	// Construct the config that should emit a warning
+	cm, err := confmaptest.LoadConf(filepath.Join("testdata", "warning-config-prometheus-relabel.yaml"))
 	require.NoError(t, err)
 	factory := NewFactory()
 	cfg := factory.CreateDefaultConfig()
 
 	sub, err := cm.Sub(component.NewIDWithName(metadata.Type, "").String())
 	require.NoError(t, err)
 	require.NoError(t, component.UnmarshalConfig(sub, cfg))
-	assert.Error(t, component.ValidateConfig(cfg))
-
+	// Use a fake logger
+	creationSet := receivertest.NewNopCreateSettings()
+	observedZapCore, observedLogs := observer.New(zap.WarnLevel)
+	creationSet.Logger = zap.New(observedZapCore)
+	_, err = createMetricsReceiver(context.Background(), creationSet, cfg, nil)
+	require.NoError(t, err)
+	// We should have received a warning
+	assert.Equal(t, 1, observedLogs.Len())
 }
 
 func TestRejectUnsupportedPrometheusFeatures(t *testing.T) {

receiver/prometheusreceiver/factory.go

@@ -5,7 +5,6 @@ package prometheusreceiver // import "github.com/open-telemetry/opentelemetry-co
 
 import (
 	"context"
-	"errors"
 
 	promconfig "github.com/prometheus/prometheus/config"
 	_ "github.com/prometheus/prometheus/discovery/install" // init() of this package registers service discovery impl.
@@ -25,8 +24,6 @@ var useCreatedMetricGate = featuregate.GlobalRegistry().MustRegister(
 		" retrieve the start time for Summary, Histogram and Sum metrics from _created metric"),
 )
 
-var errRenamingDisallowed = errors.New("metric renaming using metric_relabel_configs is disallowed")
-
 // NewFactory creates a new Prometheus receiver factory.
 func NewFactory() receiver.Factory {
 	return receiver.NewFactory(
@@ -49,5 +46,6 @@ func createMetricsReceiver(
 	cfg component.Config,
 	nextConsumer consumer.Metrics,
 ) (receiver.Metrics, error) {
+	configWarnings(set.Logger, cfg.(*Config))
 	return newPrometheusReceiver(set, cfg.(*Config), nextConsumer), nil
 }