Add PVCRef to VolumeStats

For pod volumes that reference a PVC, add a PVCRef to the corresponding
volume stat. This allows metrics to be indexed/queried by PVC name
which is more user-friendly than Pod reference

test/e2e_node/summary_test.go

@@ -181,7 +181,8 @@ var _ = framework.KubeDescribe("Summary API", func() {
 				}),
 				"VolumeStats": gstruct.MatchAllElements(summaryObjectID, gstruct.Elements{
 					"test-empty-dir": gstruct.MatchAllFields(gstruct.Fields{
-						"Name": Equal("test-empty-dir"),
+						"Name":   Equal("test-empty-dir"),
+						"PVCRef": BeNil(),
 						"FsStats": gstruct.MatchAllFields(gstruct.Fields{
 							"Time":           recent(maxStatsAge),
 							"AvailableBytes": fsCapacityBounds,