prometheus exporter ignores metrics from the Prometheus bridge (#7688)

Related to
https://github.com/open-telemetry/opentelemetry-go/issues/7686.

This makes the Prometheus exporter ignore all metrics from the
Prometheus bridge. This is almost certainly a misconfiguration, but may
happen when using autoexport or other packages that automatically
install the bridge.

CHANGELOG.md

@@ -8,6 +8,11 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Changed
+
+- `Exporter` in `go.opentelemetry.io/otel/exporter/prometheus` ignores metrics with the scope `go.opentelemetry.io/contrib/bridges/prometheus`.
+  This prevents scrape failures when the Prometheus exporter is misconfigured to get data from the Prometheus bridge. (#7688)
+
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->
 

exporters/prometheus/doc.go

@@ -4,4 +4,8 @@
 // Package prometheus provides a Prometheus Exporter that converts
 // OTLP metrics into the Prometheus exposition format and implements
 // prometheus.Collector to provide a handler for these metrics.
+//
+// The Prometheus exporter ignores metrics from the Prometheus bridge. To
+// export these metrics, simply register them directly with the Prometheus
+// Handler.
 package prometheus // import "go.opentelemetry.io/otel/exporters/prometheus"

exporters/prometheus/errors.go

@@ -7,7 +7,10 @@ import "errors"
 
 // Sentinel errors for consistent error checks in tests.
 var (
-	errInvalidMetricType = errors.New("invalid metric type")
-	errInvalidMetric     = errors.New("invalid metric")
-	errEHScaleBelowMin   = errors.New("exponential histogram scale below minimum supported")
+	errInvalidMetricType  = errors.New("invalid metric type")
+	errInvalidMetric      = errors.New("invalid metric")
+	errEHScaleBelowMin    = errors.New("exponential histogram scale below minimum supported")
+	errBridgeNotSupported = errors.New(
+		"metrics from the prometheus bridge are not supported in the prometheus exporter, and will be dropped",
+	)
 )

exporters/prometheus/exporter.go

@@ -35,6 +35,11 @@ const (
 	scopeNameLabel    = scopeLabelPrefix + "name"
 	scopeVersionLabel = scopeLabelPrefix + "version"
 	scopeSchemaLabel  = scopeLabelPrefix + "schema_url"
+	// metrics from the prometehus bridge are ignored because this produces
+	// errors. Users should directly register prometheus metrics with the
+	// Registerer, rather than round-tripping them through the bridge and
+	// exporter.
+	bridgeScopeName = "go.opentelemetry.io/contrib/bridges/prometheus"
 )
 
 var metricsPool = sync.Pool{
@@ -96,6 +101,8 @@ type collector struct {
 	unitNamer         otlptranslator.UnitNamer
 
 	inst *observ.Instrumentation
+
+	bridgeErrorOnce sync.Once
 }
 
 // New returns a Prometheus Exporter.
@@ -229,6 +236,12 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 	}
 
 	for j, scopeMetrics := range metrics.ScopeMetrics {
+		if scopeMetrics.Scope.Name == bridgeScopeName {
+			c.bridgeErrorOnce.Do(func() {
+				otel.Handle(errBridgeNotSupported)
+			})
+			continue
+		}
 		n := len(c.resourceKeyVals.keys) + 2 // resource attrs + scope name + scope version
 		kv := keyVals{
 			keys: make([]string, 0, n),

exporters/prometheus/exporter_test.go

@@ -797,6 +797,51 @@ func TestMultiScopes(t *testing.T) {
 	require.NoError(t, err)
 }
 
+func TestBridgeScopeIgnored(t *testing.T) {
+	var handledError error
+	eh := otel.ErrorHandlerFunc(func(e error) { handledError = errors.Join(handledError, e) })
+	otel.SetErrorHandler(eh)
+	ctx := t.Context()
+	registry := prometheus.NewRegistry()
+	exporter, err := New(
+		WithTranslationStrategy(otlptranslator.UnderscoreEscapingWithSuffixes),
+		WithRegisterer(registry),
+	)
+	require.NoError(t, err)
+
+	res, err := resource.New(ctx,
+		// always specify service.name because the default depends on the running OS
+		resource.WithAttributes(semconv.ServiceName("prometheus_test")),
+		// Overwrite the semconv.TelemetrySDKVersionKey value so we don't need to update every version
+		resource.WithAttributes(semconv.TelemetrySDKVersion("latest")),
+	)
+	require.NoError(t, err)
+	res, err = resource.Merge(resource.Default(), res)
+	require.NoError(t, err)
+
+	provider := metric.NewMeterProvider(
+		metric.WithReader(exporter),
+		metric.WithResource(res),
+	)
+
+	fooCounter, err := provider.Meter(bridgeScopeName, otelmetric.WithInstrumentationVersion("v0.1.0")).
+		Int64Counter(
+			"foo",
+			otelmetric.WithUnit("s"),
+			otelmetric.WithDescription("meter foo counter"))
+	assert.NoError(t, err)
+	fooCounter.Add(ctx, 100, otelmetric.WithAttributes(attribute.String("type", "foo")))
+
+	file, err := os.Open("testdata/just_target_info.txt")
+	require.NoError(t, err)
+	t.Cleanup(func() { require.NoError(t, file.Close()) })
+
+	err = testutil.GatherAndCompare(registry, file)
+	require.NoError(t, err)
+
+	require.ErrorIs(t, handledError, errBridgeNotSupported)
+}
+
 func TestDuplicateMetrics(t *testing.T) {
 	ab := attribute.NewSet(attribute.String("A", "B"))
 	withAB := otelmetric.WithAttributeSet(ab)

exporters/prometheus/testdata/just_target_info.txt

@@ -0,0 +1,3 @@
+# HELP target_info Target metadata
+# TYPE target_info gauge
+target_info{service_name="prometheus_test",telemetry_sdk_language="go",telemetry_sdk_name="opentelemetry",telemetry_sdk_version="latest"} 1