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>..
RBD Plugin: Omit volume.MetricsProvider field and add some testcases.
**What this PR does / why we need it**:
Embedded struct `volume.MetricProvider` is capitalized and should be omitted in JSON marshalling. It's also a unmarshalable struct, will cause error in `RBDUtil.loadRBD`.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
**Special notes for your reviewer**:
It's a bug I introduced in https://github.com/kubernetes/kubernetes/pull/48486. It's my bad, sorry about that.
**Release note**:
```release-note
```
3fa6761bd1