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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
```