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Volume metrics support for vSphere Cloud Provider
**What this PR does / why we need it**:
This PR adds Volume metrics support for vSphere Cloud Provider. With this we can monitor the available space, capacity and used space on volumes created using vSphere.
**Which issue(s) this PR fixes**
Fixes # https://github.com/kubernetes/kubernetes/issues/43527
Fixes # https://github.com/vmware/kubernetes/issues/208
**Special notes for your reviewer**:
Performed following steps to ensure metrics are reported correctly for vSphere Volumes.
- Created Dynamic PV using the PV claim of size 1Gi.
```
$ kubectl get pvc
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
thinclaim Bound pvc-bbc18f6a-0a04-11e8-bb1c-0050569c126b 1Gi RWO thin 5m
```
- Created Pod using this claim.
- Verified volume stats metrics.
```
/ # df -h | grep /dev/sdb
Filesystem Size Used Available Use% Mounted on
/dev/sdb 975.9M 1.3M 923.4M 0% /mnt/volume1
```
```
# HELP kubelet_volume_stats_available_bytes Number of available bytes in the volume
# TYPE kubelet_volume_stats_available_bytes gauge
kubelet_volume_stats_available_bytes{namespace="default",persistentvolumeclaim="thinclaim"} 9.68261632e+08
# HELP kubelet_volume_stats_capacity_bytes Capacity in bytes of the volume
# TYPE kubelet_volume_stats_capacity_bytes gauge
kubelet_volume_stats_capacity_bytes{namespace="default",persistentvolumeclaim="thinclaim"} 1.02330368e+09
# HELP kubelet_volume_stats_used_bytes Number of used bytes in the volume
# TYPE kubelet_volume_stats_used_bytes gauge
kubelet_volume_stats_used_bytes{namespace="default",persistentvolumeclaim="thinclaim"} 1.355776e+06
```
- Download 512MB file in the Pod on attached volume and Verified volume stats metrics is updated successfully.
```
/mnt/volume1 # df -h | grep /dev/sdb
Filesystem Size Used Available Use% Mounted on
/dev/sdb 975.9M 513.8M 410.9M 56% /mnt/volume1
```
```
# HELP kubelet_volume_stats_available_bytes Number of available bytes in the volume
# TYPE kubelet_volume_stats_available_bytes gauge
kubelet_volume_stats_available_bytes{namespace="default",persistentvolumeclaim="thinclaim"} 4.30862336e+08
# HELP kubelet_volume_stats_capacity_bytes Capacity in bytes of the volume
# TYPE kubelet_volume_stats_capacity_bytes gauge
kubelet_volume_stats_capacity_bytes{namespace="default",persistentvolumeclaim="thinclaim"} 1.02330368e+09
# HELP kubelet_volume_stats_used_bytes Number of used bytes in the volume
# TYPE kubelet_volume_stats_used_bytes gauge
kubelet_volume_stats_used_bytes{namespace="default",persistentvolumeclaim="thinclaim"} 5.38755072e+08
```
**Release note**:
```release-note
Volume metrics support for vSphere Cloud Provider
```
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