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/kube-controller-manager/app/controllermanager.go

@@ -73,6 +73,10 @@ import (
 	"k8s.io/kubernetes/pkg/serviceaccount"
 )
 
+func init() {
+	utilruntime.Must(logs.AddFeatureGates(utilfeature.DefaultMutableFeatureGate))
+}
+
 const (
 	// ControllerStartJitter is the Jitter used when starting controller managers
 	ControllerStartJitter = 1.0
@@ -118,7 +122,7 @@ controller, and serviceaccounts controller.`,
 
 			// Activate logging as soon as possible, after that
 			// show flags with the final logging configuration.
-			if err := s.Logs.ValidateAndApply(); err != nil {
+			if err := s.Logs.ValidateAndApply(utilfeature.DefaultFeatureGate); err != nil {
 				fmt.Fprintf(os.Stderr, "%v\n", err)
 				os.Exit(1)
 			}