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/kubelet/server/stats/volume_stat_calculator.go

@@ -112,7 +112,12 @@ func (s *volumeStatCalculator) calcAndStoreStats() {
 		for name, v := range blockVolumes {
 			// Only add the blockVolume if it implements the MetricsProvider interface
 			if _, ok := v.(volume.MetricsProvider); ok {
-				metricVolumes[name] = v
+				// Some drivers inherit the MetricsProvider interface from Filesystem
+				// mode volumes, but do not implement it for Block mode. Checking
+				// SupportsMetrics() will prevent panics in that case.
+				if v.SupportsMetrics() {
+					metricVolumes[name] = v
+				}
 			}
 		}
 	}

pkg/kubelet/server/stats/volume_stat_calculator_test.go

@@ -246,6 +246,8 @@ func (v *fakeBlockVolume) GetGlobalMapPath(*volume.Spec) (string, error) { retur
 
 func (v *fakeBlockVolume) GetPodDeviceMapPath() (string, string) { return "", "" }
 
+func (v *fakeBlockVolume) SupportsMetrics() bool { return true }
+
 func (v *fakeBlockVolume) GetMetrics() (*volume.Metrics, error) {
 	return expectedBlockMetrics(), nil
 }