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/metrics_nil.go

@@ -23,6 +23,11 @@ var _ MetricsProvider = &MetricsNil{}
 // metrics.
 type MetricsNil struct{}
 
+// SupportsMetrics returns false for the MetricsNil type.
+func (*MetricsNil) SupportsMetrics() bool {
+	return false
+}
+
 // GetMetrics returns an empty Metrics and an error.
 // See MetricsProvider.GetMetrics
 func (*MetricsNil) GetMetrics() (*Metrics, error) {