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/metrics_nil.go

@@ -0,0 +1,31 @@
+/*
+Copyright 2014 The Kubernetes Authors All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package volume
+
+import "errors"
+
+var _ MetricsProvider = &MetricsNil{}
+
+// MetricsNil represents a MetricsProvider that does not support returning
+// Metrics.  It serves as a placeholder for Volumes that do not yet support metrics.
+type MetricsNil struct{}
+
+// See MetricsProvider.GetMetrics
+// GetMetrics returns an empty Metrics and an error.
+func (*MetricsNil) GetMetrics() (*Metrics, error) {
+	return &Metrics{}, errors.New("metrics are not supported for MetricsNil Volumes")
+}