Automatic merge from submit-queue (batch tested with PRs 49081, 49318, 49219, 48989, 48486)
Use MetricsStatsFs to expose RBD volume plugin metrics.
**What this PR does / why we need it**:
We need to monitor RBD volume usage of our cluster and configure alerts if RBD volume is nearly full. Users of cluster also need to see usage history graph on Grafana.
This PR use MetricsStatsFs to implement MetricsProvider interface of RBD plugin (same as `gce_pd`), so kubelet /stat/summary can expose RBD volume stats.
**Special notes for your reviewer**:
cc @rootfs
**Release note**:
```release-note
NONE
```
ea89d62710