feat(ccm): introduce metric route_controller_route_sync_total

This metric tracks the amount of times routes are synced with the
cloud provider. This metric is mainly used for A/B tests on the
feature gate `CloudControllerManagerWatchBasedRoutesReconciliation`
introduced in Kubernetes v1.35.

staging/src/k8s.io/cloud-provider/controllers/route/metrics.go

@@ -0,0 +1,30 @@
+package route
+
+import (
+	"sync"
+
+	"k8s.io/component-base/metrics"
+	"k8s.io/component-base/metrics/legacyregistry"
+)
+
+const (
+	// subsystem is the name of this subsystem used for prometheus metrics.
+	subsystem = "route_controller"
+)
+
+var registration sync.Once
+
+var (
+	routeSyncCount = metrics.NewCounter(&metrics.CounterOpts{
+		Name:           "route_sync_total",
+		Subsystem:      subsystem,
+		Help:           "A metric counting the amount of times routes have been synced with the cloud provider.",
+		StabilityLevel: metrics.BETA,
+	})
+)
+
+func registerMetrics() {
+	registration.Do(func() {
+		legacyregistry.MustRegister(routeSyncCount)
+	})
+}

staging/src/k8s.io/cloud-provider/controllers/route/route_controller.go

@@ -87,6 +87,8 @@ func New(
 	clusterName string,
 	clusterCIDRs []*net.IPNet,
 ) (*RouteController, error) {
+	registerMetrics()
+
 	if len(clusterCIDRs) == 0 {
 		klog.Fatal("RouteController: Must specify clusterCIDR.")
 	}
@@ -247,6 +249,8 @@ func (rc *RouteController) processNextWorkItem(ctx context.Context) bool {
 }
 
 func (rc *RouteController) reconcileNodeRoutes(ctx context.Context) error {
+	routeSyncCount.Inc()
+
 	routeList, err := rc.routes.ListRoutes(ctx, rc.clusterName)
 	if err != nil {
 		return fmt.Errorf("error listing routes: %v", err)