630c780f37
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 ```