Automatic merge from submit-queue (batch tested with PRs 51335, 51364, 51130, 48075, 50920)
Graduate custom metrics API to v1beta1
This graduates custom-metrics.metrics.k8s.io/v1alpha1 to custom-metrics.metrics.k8s.io/v1beta1. The move is more-or-less just a straightforward rename.
Part of kubernetes/features#117 and kubernetes/features#118
```release-note
the custom metrics API (custom-metrics.metrics.k8s.io) has moved from v1alpha1 to v1beta1
```