logging: add ContextualLogging feature

InitLogs overrides the klog default and turns contextual logging off. This
ensures that it is only enabled in Kubernetes commands that explicitly enable
it via a feature gate. A feature gate for it gets defined in
k8s.io/component-base/logs and is then used by Options.ValidateAndApply.

The effect of disabling contextual logging is very limited according to
benchmarks with kube-scheduler. The feature gets added anyway to satisfy the
PRR recommendation that features should be controllable.

The following commands have support for contextual logging:
- kube-apiserver
- kube-controller-manager
- kubelet
- kube-scheduler
- component-base/logs example

Supporting a feature gate check in ValidateAndApply and not in InitLogs is a
simplification: changing InitLogs to accept a FeatureGate would have implied
changing also component-base/cli.Run. This didn't seem worthwhile because
ValidateAndApply already covers the relevant commands.

cmd/kubelet/app/server.go

@@ -45,6 +45,7 @@ import (
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/types"
 	utilnet "k8s.io/apimachinery/pkg/util/net"
+	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
 	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/apimachinery/pkg/util/wait"
 	genericapiserver "k8s.io/apiserver/pkg/server"
@@ -103,6 +104,10 @@ import (
 	netutils "k8s.io/utils/net"
 )
 
+func init() {
+	utilruntime.Must(logs.AddFeatureGates(utilfeature.DefaultMutableFeatureGate))
+}
+
 const (
 	// Kubelet component name
 	componentKubelet = "kubelet"
@@ -226,7 +231,7 @@ HTTP server: The kubelet can also listen for HTTP and respond to a simple API
 			// Config and flags parsed, now we can initialize logging.
 			logs.InitLogs()
 			logOption := &logs.Options{Config: kubeletConfig.Logging}
-			if err := logOption.ValidateAndApply(); err != nil {
+			if err := logOption.ValidateAndApply(utilfeature.DefaultFeatureGate); err != nil {
 				klog.ErrorS(err, "Failed to initialize logging")
 				os.Exit(1)
 			}