Merge pull request #66333 from hzxuzhonghu/audit

Automatic merge from submit-queue (batch tested with PRs 66333, 65542). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add String method to audit.Backend interface

**What this PR does / why we need it**:

Add `String()` method to `audit.Backend` interface.  Should enforce backend to implement this method.
Because https://github.com/kubernetes/kubernetes/blob/master/staging/src/k8s.io/apiserver/pkg/audit/union.go#L65

We encountered this issue when we upgrade recently and we implemented our private backend.

**Release note**:

```release-note
NONE
```

staging/src/k8s.io/apiserver/pkg/audit/types.go

@@ -39,4 +39,7 @@ type Backend interface {
 	// events are delivered. It can be assumed that this method is called after
 	// the stopCh channel passed to the Run method has been closed.
 	Shutdown()
+
+	// Returns the backend PluginName.
+	String() string
 }

staging/src/k8s.io/apiserver/pkg/audit/union_test.go

@@ -40,6 +40,10 @@ func (f *fakeBackend) Shutdown() {
 	// Nothing to do here.
 }
 
+func (f *fakeBackend) String() string {
+	return ""
+}
+
 func TestUnion(t *testing.T) {
 	backends := []Backend{
 		new(fakeBackend),

staging/src/k8s.io/apiserver/plugin/pkg/audit/fake/fake.go

@@ -44,3 +44,7 @@ func (b *Backend) ProcessEvents(ev ...*auditinternal.Event) {
 		b.OnRequest(ev)
 	}
 }
+
+func (b *Backend) String() string {
+	return ""
+}

test/integration/examples/webhook_test.go

@@ -111,8 +111,14 @@ type auditSinkFunc func(events ...*auditinternal.Event)
 func (f auditSinkFunc) ProcessEvents(events ...*auditinternal.Event) {
 	f(events...)
 }
+
 func (auditSinkFunc) Run(stopCh <-chan struct{}) error {
 	return nil
 }
+
 func (auditSinkFunc) Shutdown() {
 }
+
+func (auditSinkFunc) String() string {
+	return ""
+}