Do not error when facing duplicate metrics + log with component

lib/service/service.go

@@ -3416,6 +3416,16 @@ func (process *TeleportProcess) initUploaderService() error {
 	return nil
 }
 
+// promHTTPLogAdapter adapts a slog.Logger into a promhttp.Logger.
+type promHTTPLogAdapter struct {
+	*slog.Logger
+}
+
+// Println implements the promhttp.Logger interface.
+func (l promHTTPLogAdapter) Println(v ...interface{}) {
+	l.Error(fmt.Sprint(v...))
+}
+
 // initMetricsService starts the metrics service currently serving metrics for
 // prometheus consumption
 func (process *TeleportProcess) initMetricsService() error {
@@ -3424,12 +3434,20 @@ func (process *TeleportProcess) initMetricsService() error {
 	// We gather metrics both from the in-process registry (preferred metrics registration method)
 	// and the global registry (used by some Teleport services and many dependencies).
 	gatherers := prometheus.Gatherers{
-		prometheus.DefaultGatherer,
 		process.metricsRegistry,
+		prometheus.DefaultGatherer,
 	}
 
 	metricsHandler := promhttp.InstrumentMetricHandler(
-		process.metricsRegistry, promhttp.HandlerFor(gatherers, promhttp.HandlerOpts{}),
+		process.metricsRegistry, promhttp.HandlerFor(gatherers, promhttp.HandlerOpts{
+			// Errors can happen if metrics are registered with identical names in both the local and the global registry.
+			// In this case, we log the error but continue collecting metrics. The first collected metric will win
+			// (the one from the local metrics registry takes precedence).
+			// As we move more things to the local registry, especially in other tools like tbot, we will have less
+			// conflicts in tests.
+			ErrorHandling: promhttp.ContinueOnError,
+			ErrorLog:      promHTTPLogAdapter{process.logger.With(teleport.ComponentKey, teleport.ComponentMetrics)},
+		}),
 	)
 
 	mux.Handle("/metrics", metricsHandler)

lib/service/service_test.go

@@ -2020,19 +2020,6 @@ func TestMetricsService(t *testing.T) {
 	metricsURL.Path = "/metrics"
 	resp, err := http.Get(metricsURL.String())
 	require.NoError(t, err)
-	// WIP: the test is failing in the CI only. This piece of code can be
-	// ignored and will be removed from the PR once I know why the HTTP status
-	// code is 500.
-	if resp.StatusCode != 200 {
-		t.Log("Non 200 status code received")
-		body, err := io.ReadAll(resp.Body)
-		if err != nil {
-			t.Logf("Cannot read body: %s", err.Error())
-		} else {
-			t.Logf("Body: %s", string(body))
-		}
-		t.Logf("Header: %#v", resp.Header)
-	}
 	require.Equal(t, http.StatusOK, resp.StatusCode)
 
 	body, err := io.ReadAll(resp.Body)