Add SupportsMetrics() for Block-mode volumes

Volumes that are provisioned with `VolumeMode: Block` often have a
MetrucsProvider interface declared in their type. However, the
MetricsProvider should implement a GetMetrics() function. In the cases
where the storage drivers do not implement GetMetrics(), a panic can
occur.

Usual type-assertions are not sufficient in this case. All assertions
assume the interface is present. There is no straight forward way to
verify that a valid GetMetrics() function is provided.

By adding SupportsMetrics(), storage driver implementations require
careful reviewing for metrics support.

pkg/volume/rbd/rbd.go

@@ -938,6 +938,12 @@ func (rbd *rbd) rbdPodDeviceMapPath() (string, string) {
 	return rbd.plugin.host.GetPodVolumeDeviceDir(rbd.podUID, utilstrings.EscapeQualifiedName(name)), rbd.volName
 }
 
+// SupportsMetrics returns true for rbdDiskMapper as it initializes the
+// MetricsProvider.
+func (rdm *rbdDiskMapper) SupportsMetrics() bool {
+	return true
+}
+
 type rbdDiskUnmapper struct {
 	*rbdDiskMapper
 }