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/nfs/nfs_test.go

@@ -92,6 +92,7 @@ func newMockRecycler(spec *volume.Spec, host volume.VolumeHost, config volume.Vo
 type mockRecycler struct {
 	path string
 	host volume.VolumeHost
+	volume.MetricsNil
 }
 
 func (r *mockRecycler) GetPath() string {