Automatic merge from submit-queue. 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>.

move prometheus register to pkg/endpoints/metrics/metrics.go

**What this PR does / why we need it**:
delete  k8s.io/apiserver/pkg/endpoints/apiserver.go
move prometheus init from k8s.io/apiserver/pkg/endpoints/apiserver.go to k8s.io/apiserver/pkg/endpoints/metrics/metrics.go

Since  k8s.io/apiserver/pkg/endpoints/apiserver.go only has a metrics init func, which should be in metrics pkg.

**Release note**:

```release-note
NONE
```

/assign @sttts @liggitt