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