Volume Metrics Interface and base implementation.

- Add volume.MetricsProvider function to Volume interface.
- Add volume.MetricsDu for providing metrics via executing "du".
- Add volulme.MetricsNil for unsupported Volumes.

pkg/volume/git_repo/git_repo.go

@@ -89,6 +89,7 @@ type gitRepoVolume struct {
 	volName string
 	podUID  types.UID
 	plugin  *gitRepoPlugin
+	volume.MetricsNil
 }
 
 var _ volume.Volume = &gitRepoVolume{}