Add fallback metric producer factory

Signed-off-by: gouthamve <[email protected]>

exporters/autoexport/metrics.go

@@ -79,6 +79,12 @@ func RegisterMetricProducer(name string, factory func(context.Context) (metric.P
 	must(metricsProducers.registry.store(name, factory))
 }
 
+// WithFallbackMetricProducer sets the fallback producer to use when no producer
+// is configured through the OTEL_METRICS_PRODUCERS environment variable.
+func WithFallbackMetricProducer(producerFactory func(ctx context.Context) (metric.Producer, error)) {
+	metricsProducers.fallbackProducer = producerFactory
+}
+
 var (
 	metricsSignal    = newSignal[metric.Reader]("OTEL_METRICS_EXPORTER")
 	metricsProducers = newProducerRegistry("OTEL_METRICS_PRODUCERS")
@@ -217,8 +223,9 @@ func getenv(key, fallback string) string {
 }
 
 type producerRegistry struct {
-	envKey   string
-	registry *registry[metric.Producer]
+	envKey           string
+	fallbackProducer func(context.Context) (metric.Producer, error)
+	registry         *registry[metric.Producer]
 }
 
 func newProducerRegistry(envKey string) producerRegistry {
@@ -233,6 +240,10 @@ func newProducerRegistry(envKey string) producerRegistry {
 func (pr producerRegistry) create(ctx context.Context) (metric.Producer, error) {
 	expType := os.Getenv(pr.envKey)
 	if expType == "" {
+		if pr.fallbackProducer != nil {
+			return pr.fallbackProducer(ctx)
+		}
+
 		return nil, nil
 	}
 

exporters/autoexport/metrics_test.go

@@ -15,9 +15,11 @@ import (
 	"testing"
 
 	"go.opentelemetry.io/collector/pdata/pmetric/pmetricotlp"
+	prometheusbridge "go.opentelemetry.io/contrib/bridges/prometheus"
 	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/sdk/metric"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/stretchr/testify/assert"
 	"go.uber.org/goleak"
 )
@@ -192,3 +194,42 @@ func TestMetricProducerPrometheusWithPrometheusExporter(t *testing.T) {
 	assert.NoError(t, mp.Shutdown(context.Background()))
 	goleak.VerifyNone(t)
 }
+
+func TestMetricProducerFallbackWithPrometheusExporter(t *testing.T) {
+	assertNoOtelHandleErrors(t)
+
+	reg := prometheus.NewRegistry()
+	someDummyMetric := prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "dummy_metric",
+		Help: "dummy metric",
+	})
+	reg.MustRegister(someDummyMetric)
+
+	WithFallbackMetricProducer(func(context.Context) (metric.Producer, error) {
+		return prometheusbridge.NewMetricProducer(prometheusbridge.WithGatherer(reg)), nil
+	})
+
+	t.Setenv("OTEL_METRICS_EXPORTER", "prometheus")
+	t.Setenv("OTEL_EXPORTER_PROMETHEUS_PORT", "0")
+
+	r, err := NewMetricReader(context.Background())
+	assert.NoError(t, err)
+
+	// pull-based exporters like Prometheus need to be registered
+	mp := metric.NewMeterProvider(metric.WithReader(r))
+
+	rws, ok := r.(readerWithServer)
+	if !ok {
+		t.Errorf("expected readerWithServer but got %v", r)
+	}
+
+	resp, err := http.Get(fmt.Sprintf("http://%s/metrics", rws.addr))
+	assert.NoError(t, err)
+	body, err := io.ReadAll(resp.Body)
+	assert.NoError(t, err)
+
+	assert.Contains(t, string(body), "HELP dummy_metric_total dummy metric")
+
+	assert.NoError(t, mp.Shutdown(context.Background()))
+	goleak.VerifyNone(t)
+}