[fix] Use provided registry in metrics.RegisterCollectors (#64919)

lib/observability/metrics/prometheus.go

@@ -42,7 +42,7 @@ func RegisterPrometheusCollectors(collectors ...prometheus.Collector) error {
 func RegisterCollectors(registry prometheus.Registerer, collectors ...prometheus.Collector) error {
 	var errs []error
 	for _, c := range collectors {
-		if err := prometheus.Register(c); err != nil {
+		if err := registry.Register(c); err != nil {
 			var alreadyRegisteredError prometheus.AlreadyRegisteredError
 			if errors.As(err, &alreadyRegisteredError) {
 				continue

lib/observability/metrics/prometheus_test.go

@@ -0,0 +1,101 @@
+// Teleport
+// Copyright (C) 2026 Gravitational, Inc.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+package metrics
+
+import (
+	"slices"
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCollectorRegistration(t *testing.T) {
+	// Create two metrics that will be registered with different
+	// registries.
+	metric1 := prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "test",
+		Subsystem: "metrics",
+		Name:      "fake1",
+		Help:      "not a real metric",
+	}, []string{"test"})
+	metric1.WithLabelValues("test").Add(1)
+
+	metric2 := prometheus.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "test",
+		Subsystem: "metrics",
+		Name:      "fake2",
+		Help:      "not a real metric",
+	}, []string{"test"})
+	metric2.WithLabelValues("test").Add(1)
+
+	// Validate that neither metric is present in the global registry.
+	metrics, err := prometheus.DefaultGatherer.Gather()
+	require.NoError(t, err)
+	require.False(t, slices.ContainsFunc(metrics, func(m *dto.MetricFamily) bool {
+		return m.Name != nil && (*m.Name == "test_metrics_fake1" || *m.Name == "test_metrics_fake2")
+	}))
+
+	// Validate that no metrics are present in the test registry.
+	registry := prometheus.NewRegistry()
+	metrics, err = registry.Gather()
+	require.NoError(t, err)
+	require.Empty(t, metrics)
+
+	// Register metric1 with only the test registry.
+	require.NoError(t, RegisterCollectors(registry, metric1))
+
+	// Validate that metric1 is not present in the global registry.
+	metrics, err = prometheus.DefaultGatherer.Gather()
+	require.NoError(t, err)
+	require.False(t, slices.ContainsFunc(metrics, func(m *dto.MetricFamily) bool {
+		return m.Name != nil && (*m.Name == "test_metrics_fake1" || *m.Name == "test_metrics_fake2")
+	}))
+
+	// Validate that metric1 is present in the test registry.
+	metrics, err = registry.Gather()
+	require.NoError(t, err)
+	require.Len(t, metrics, 1)
+	require.True(t, slices.ContainsFunc(metrics, func(m *dto.MetricFamily) bool {
+		return m.Name != nil && *m.Name == "test_metrics_fake1"
+	}))
+
+	// Register metric2 with only the global registry.
+	require.NoError(t, RegisterPrometheusCollectors(metric2))
+	// Remove the metric to prevent polluting the default registry so
+	// that running with -count > 1 does not fail.
+	t.Cleanup(func() { _ = prometheus.DefaultRegisterer.Unregister(metric2) })
+
+	// Validate that metric2 is present in the global registry.
+	metrics, err = prometheus.DefaultGatherer.Gather()
+	require.NoError(t, err)
+	require.True(t, slices.ContainsFunc(metrics, func(m *dto.MetricFamily) bool {
+		return m.Name != nil && *m.Name == "test_metrics_fake2"
+	}))
+
+	// Validate that metric2 is not present in the test registry.
+	metrics, err = registry.Gather()
+	require.NoError(t, err)
+	require.Len(t, metrics, 1)
+	require.False(t, slices.ContainsFunc(metrics, func(m *dto.MetricFamily) bool {
+		return m.Name != nil && *m.Name == "test_metrics_fake2"
+	}))
+
+	// Validate that registering the same metric twice doesn't produce an error.
+	require.NoError(t, RegisterCollectors(registry, metric1))
+	require.NoError(t, RegisterPrometheusCollectors(metric2))
+}