Add producer support for Prometheus exporter as well

Signed-off-by: gouthamve <gouthamve@gmail.com>

exporters/autoexport/metrics.go

@@ -136,10 +136,22 @@ func init() {
 	})
 	RegisterMetricReader("prometheus", func(ctx context.Context) (metric.Reader, error) {
 		// create an isolated registry instead of using the global registry --
-		// the user might not want to mix OTel with non-OTel metrics
+		// the user might not want to mix OTel with non-OTel metrics.
+		// Those that want to comingle metrics from global registry can use
+		// OTEL_METRICS_PRODUCERS=prometheus
 		reg := prometheus.NewRegistry()
 
-		reader, err := promexporter.New(promexporter.WithRegisterer(reg))
+		exporterOpts := []promexporter.Option{promexporter.WithRegisterer(reg)}
+
+		producer, err := metricsProducers.create(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if producer != nil {
+			exporterOpts = append(exporterOpts, promexporter.WithProducer(producer))
+		}
+
+		reader, err := promexporter.New(exporterOpts...)
 		if err != nil {
 			return nil, err
 		}

exporters/autoexport/metrics_test.go

@@ -11,6 +11,7 @@ import (
 	"net/http/httptest"
 	"reflect"
 	"runtime/debug"
+	"strings"
 	"testing"
 
 	"go.opentelemetry.io/collector/pdata/pmetric/pmetricotlp"
@@ -122,7 +123,7 @@ func TestMetricExporterPrometheusInvalidPort(t *testing.T) {
 	assert.ErrorContains(t, err, "binding")
 }
 
-func TestMetricProducerPrometheus(t *testing.T) {
+func TestMetricProducerPrometheusWithOTLPExporter(t *testing.T) {
 	assertNoOtelHandleErrors(t)
 
 	requestWaitChan := make(chan struct{})
@@ -150,12 +151,44 @@ func TestMetricProducerPrometheus(t *testing.T) {
 	assert.IsType(t, &metric.PeriodicReader{}, r)
 
 	// Register it with a meter provider to ensure it is used.
-	metric.NewMeterProvider(metric.WithReader(r))
+	mp := metric.NewMeterProvider(metric.WithReader(r))
 
 	// Shutdown actually makes an export call.
 	assert.NoError(t, r.Shutdown(context.Background()))
 
 	<-requestWaitChan
 	ts.Close()
+	assert.NoError(t, mp.Shutdown(context.Background()))
+	goleak.VerifyNone(t)
+}
+
+func TestMetricProducerPrometheusWithPrometheusExporter(t *testing.T) {
+	assertNoOtelHandleErrors(t)
+
+	t.Setenv("OTEL_METRICS_EXPORTER", "prometheus")
+	t.Setenv("OTEL_EXPORTER_PROMETHEUS_PORT", "0")
+	t.Setenv("OTEL_METRICS_PRODUCERS", "prometheus")
+
+	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)
+
+	// By default there are two metrics exporter. target_info and promhttp_metric_handler_errors_total.
+	// But by including the prometheus producer we should have more.
+	assert.Greater(t, strings.Count(string(body), "# HELP"), 2)
+
+	assert.NoError(t, mp.Shutdown(context.Background()))
 	goleak.VerifyNone(t)
 }