pkg/kubelet: add kubelet_metrics_provider metric

This metric is meant to help end-users identify which metrics provider
kubelet is using under the hood to expose container stats.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

pkg/kubelet/server/metrics/metrics.go

@@ -26,6 +26,9 @@ import (
 
 const (
 	kubeletSubsystem = "kubelet"
+
+	CAdvisorMetricsProvider MetricsProviderType = "cadvisor"
+	CRIMetricsProvider      MetricsProviderType = "cri"
 )
 
 var (
@@ -76,6 +79,15 @@ var (
 		},
 		[]string{"metric_source"},
 	)
+	MetricsProvider = metrics.NewGaugeVec(
+		&metrics.GaugeOpts{
+			Subsystem:      kubeletSubsystem,
+			Name:           "metrics_provider",
+			Help:           "Metrics provider used by kubelet to collect container stats. Values can be 'cadvisor' and 'cri'",
+			StabilityLevel: metrics.ALPHA,
+		},
+		[]string{"provider"},
+	)
 )
 
 var registerMetrics sync.Once
@@ -87,6 +99,7 @@ func Register() {
 		legacyregistry.MustRegister(HTTPRequestsDuration)
 		legacyregistry.MustRegister(HTTPInflightRequests)
 		legacyregistry.MustRegister(VolumeStatCalDuration)
+		legacyregistry.MustRegister(MetricsProvider)
 	})
 }
 
@@ -99,3 +112,11 @@ func SinceInSeconds(start time.Time) float64 {
 func CollectVolumeStatCalDuration(metricSource string, start time.Time) {
 	VolumeStatCalDuration.WithLabelValues(metricSource).Observe(SinceInSeconds(start))
 }
+
+type MetricsProviderType string
+
+// SetMetricsProvider sets the metrics provider.
+func SetMetricsProvider(provider MetricsProviderType) {
+	MetricsProvider.Reset()
+	MetricsProvider.WithLabelValues(string(provider)).Set(1)
+}

pkg/kubelet/server/metrics/metrics_test.go

@@ -0,0 +1,65 @@
+/*
+Copyright The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package metrics
+
+import (
+	"strings"
+	"testing"
+
+	"k8s.io/component-base/metrics/legacyregistry"
+	"k8s.io/component-base/metrics/testutil"
+)
+
+func TestSetMetricsProvider(t *testing.T) {
+	Register()
+	MetricsProvider.Reset()
+
+	tests := []struct {
+		name     string
+		provider MetricsProviderType
+		want     string
+	}{
+		{
+			name:     "cadvisor provider",
+			provider: CAdvisorMetricsProvider,
+			want: `
+# HELP kubelet_metrics_provider [ALPHA] Metrics provider used by kubelet to collect container stats. Values can be 'cadvisor' and 'cri'
+# TYPE kubelet_metrics_provider gauge
+kubelet_metrics_provider{provider="cadvisor"} 1
+`,
+		},
+		{
+			name:     "cri provider",
+			provider: CRIMetricsProvider,
+			want: `
+# HELP kubelet_metrics_provider [ALPHA] Metrics provider used by kubelet to collect container stats. Values can be 'cadvisor' and 'cri'
+# TYPE kubelet_metrics_provider gauge
+kubelet_metrics_provider{provider="cri"} 1
+`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			SetMetricsProvider(tt.provider)
+
+			if err := testutil.GatherAndCompare(legacyregistry.DefaultGatherer, strings.NewReader(tt.want), "kubelet_metrics_provider"); err != nil {
+				t.Errorf("unexpected metric output: %v", err)
+			}
+		})
+	}
+}

pkg/kubelet/server/server.go

@@ -476,6 +476,7 @@ func (s *Server) InstallAuthNotRequiredHandlers(ctx context.Context) {
 	r.RawMustRegister(metrics.NewPrometheusMachineCollector(prometheusHostAdapter{s.host}, includedMetrics))
 	if utilfeature.DefaultFeatureGate.Enabled(features.PodAndContainerStatsFromCRI) {
 		r.CustomRegister(collectors.NewCRIMetricsCollector(context.TODO(), s.host.ListPodSandboxMetrics, s.host.ListMetricDescriptors))
+		servermetrics.SetMetricsProvider(servermetrics.CRIMetricsProvider)
 	} else {
 		cadvisorOpts := cadvisorv2.RequestOptions{
 			IdType:    cadvisorv2.TypeName,
@@ -483,6 +484,7 @@ func (s *Server) InstallAuthNotRequiredHandlers(ctx context.Context) {
 			Recursive: true,
 		}
 		r.RawMustRegister(metrics.NewPrometheusCollector(prometheusHostAdapter{s.host}, containerPrometheusLabelsFunc(s.host), includedMetrics, clock.RealClock{}, cadvisorOpts))
+		servermetrics.SetMetricsProvider(servermetrics.CAdvisorMetricsProvider)
 	}
 	s.restfulCont.Handle(cadvisorMetricsPath,
 		compbasemetrics.HandlerFor(r, compbasemetrics.HandlerOpts{ErrorHandling: compbasemetrics.ContinueOnError}),