Proposal for adding PVC info to VolumeStats

Flushes out details for part 1 of the changes described in
kubernetes/community#855

Feature: kubernetes/enhancements#363

volume_stats_pvc_ref.md

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+# Add PVC reference in Volume Stats
+
+## Background
+Pod volume stats tracked by kubelet do not currently include any information about the PVC (if the pod volume was referenced via a PVC)
+
+This prevents exposing (and querying) volume metrics labeled by PVC name which is preferable for users, given that PVC is a top-level API object.
+
+## Proposal
+
+Modify ```VolumeStats``` tracked in Kubelet and populate with PVC info:
+
+```
+// VolumeStats contains data about Volume filesystem usage.
+type VolumeStats struct {
+	// Embedded FsStats
+	FsStats
+	// Name is the name given to the Volume
+	// +optional
+	Name string `json:"name,omitempty"`
++	// PVCRef is a reference to the measured PVC.
++	// +optional
++	PVCRef PVCReference `json:"pvcRef"`
+}
+
++// PVCReference contains enough information to describe the referenced PVC.
++type PVCReference struct {
++	Name      string `json:"name"`
++	Namespace string `json:"namespace"`
++}
+```
+
+## Implementation
+2 options are described below. Option 1 supports current requirements/requested use cases. Option 2 supports an additional use case that was being discussed and is called out for completeness/discussion/feedback.
+
+### Option 1
+- Modify ```kubelet::server::stats::calcAndStoreStats()```
+    - If the pod volume is referenced via a PVC, populate ```PVCRef``` in VolumeStats using the Pod spec
+
+    - The Pod spec is already available in this method, so the changes are contained to this function.
+
+- The limitation of this approach is that we're limited to reporting only what is available in the pod spec (Pod namespace and PVC claimname)
+
+### Option 2
+- Modify the ```volumemanager::GetMountedVolumesForPod()``` (or add a new function) to return additional volume information from the ASOW/DSOW caches
+    - Use this to populate PVCRef in VolumeStats
+
+- This allows us to get information not available in the Pod spec such as the PV name/UID which can be used to label metrics - enables exposing/querying volume metrics by PV name
+- It's unclear whether this is a use case we need to/should support:
+  * Volume metrics are only refreshed for mounted volumes which implies a bound/available PVC
+  * We expect most user-storage interactions to be via the PVC
+
+
+
+