General: metrics server expose apiserver metrics

Signed-off-by: Bojan Zelic <bnzelic@gmail.com>

CHANGELOG.md

@@ -61,6 +61,7 @@ To learn more about active deprecations, we recommend checking [GitHub Discussio
 - **General**: Adding a changelog validating script to check for formatting and order ([#3190](https://github.com/kedacore/keda/issues/3190))
 - **General**: Update golangci-lint version documented in CONTRIBUTING.md since old version doesn't support go 1.20 (N/A)
 - **General**: Updated AWS SDK and updated all the aws scalers ([#4905](https://github.com/kedacore/keda/issues/4905))
+- **General**: Add apiserver prometheus metrics to keda metric server ([#4460](https://github.com/kedacore/keda/issues/4460))
 - **Azure Pod Identity**: Introduce validation to prevent usage of empty identity ID for Azure identity providers ([#4528](https://github.com/kedacore/keda/issues/4528))
 - **Prometheus Scaler**: Remove trailing whitespaces in customAuthHeader and customAuthValue ([#4960](https://github.com/kedacore/keda/issues/4960))
 - **Pulsar Scaler**: Add support for OAuth extensions ([#4700](https://github.com/kedacore/keda/issues/4700))

cmd/adapter/main.go

@@ -20,22 +20,29 @@ import (
 	"context"
 	"flag"
 	"fmt"
+	"net/http"
 	"os"
 
 	appsv1 "k8s.io/api/apps/v1"
+	apimetrics "k8s.io/apiserver/pkg/endpoints/metrics"
 	"k8s.io/client-go/kubernetes/scheme"
+	kubemetrics "k8s.io/component-base/metrics"
+	"k8s.io/component-base/metrics/legacyregistry"
 	"k8s.io/klog/v2"
 	"k8s.io/klog/v2/klogr"
 	ctrl "sigs.k8s.io/controller-runtime"
 	ctrlcache "sigs.k8s.io/controller-runtime/pkg/cache"
+	ctrlmetrics "sigs.k8s.io/controller-runtime/pkg/metrics"
 	"sigs.k8s.io/controller-runtime/pkg/metrics/server"
+
 	basecmd "sigs.k8s.io/custom-metrics-apiserver/pkg/cmd"
 	"sigs.k8s.io/custom-metrics-apiserver/pkg/provider"
 
 	kedav1alpha1 "github.com/kedacore/keda/v2/apis/keda/v1alpha1"
 	"github.com/kedacore/keda/v2/pkg/metricsservice"
 	kedaprovider "github.com/kedacore/keda/v2/pkg/provider"
 	kedautil "github.com/kedacore/keda/v2/pkg/util"
+	"github.com/prometheus/client_golang/prometheus/collectors"
 )
 
 // Adapter creates External Metrics Provider
@@ -96,10 +103,9 @@ func (a *Adapter) makeProvider(ctx context.Context) (provider.ExternalMetricsPro
 	cfg.Burst = adapterClientRequestBurst
 	cfg.DisableCompression = disableCompression
 
-	metricsBindAddress := fmt.Sprintf(":%v", metricsAPIServerPort)
 	mgr, err := ctrl.NewManager(cfg, ctrl.Options{
 		Metrics: server.Options{
-			BindAddress: metricsBindAddress,
+			BindAddress: "0", // disabled since we use our own server to serve metrics
 		},
 		Scheme: scheme,
 		Cache: ctrlcache.Options{
@@ -131,6 +137,40 @@ func (a *Adapter) makeProvider(ctx context.Context) (provider.ExternalMetricsPro
 	return kedaprovider.NewProvider(ctx, logger, mgr.GetClient(), *grpcClient), stopCh, nil
 }
 
+func getMetricHandler() http.HandlerFunc {
+	// Register apiserver metrics in legacy registry
+	// this contains the apiserver_* metrics
+	apimetrics.Register()
+
+	// unregister duplicate collectors that are already handled by controller-runtime's registry
+	legacyregistry.Registerer().Unregister(collectors.NewProcessCollector(collectors.ProcessCollectorOpts{}))
+	legacyregistry.Registerer().Unregister(collectors.NewGoCollector(collectors.WithGoCollectorRuntimeMetrics(collectors.MetricsAll)))
+
+	// Return handler that serves metrics from both legacy and controller-runtime registry
+	return func(w http.ResponseWriter, req *http.Request) {
+		legacyregistry.Handler().ServeHTTP(w, req)
+
+		kubemetrics.HandlerFor(ctrlmetrics.Registry, kubemetrics.HandlerOpts{}).ServeHTTP(w, req)
+	}
+}
+
+func RunMetricsServer() {
+	h := getMetricHandler()
+	mux := http.NewServeMux()
+	mux.Handle("/metrics", h)
+
+	metricsBindAddress := fmt.Sprintf(":%v", metricsAPIServerPort)
+
+	go func() {
+		logger.Info("starting /metrics server endpoint")
+		// nosemgrep: use-tls
+		err := http.ListenAndServe(metricsBindAddress, mux)
+		if err != nil {
+			panic(err)
+		}
+	}()
+}
+
 // generateDefaultMetricsServiceAddr generates default Metrics Service gRPC Server address based on the current Namespace.
 // By default the Metrics Service gRPC Server runs in the same namespace on the keda-operator pod.
 func generateDefaultMetricsServiceAddr() string {
@@ -196,6 +236,8 @@ func main() {
 	cmd.WithExternalMetrics(kedaProvider)
 
 	logger.Info(cmd.Message)
+
+	RunMetricsServer()
 	if err = cmd.Run(stopCh); err != nil {
 		return
 	}