spec: add optional secrets to NodeGetVolumeStatsRequest

For BlockVolume type of volumes, there are very little stats that can be
gathered. The only reasonable metric on the Node that is available
without knowing the contents of the BlockVolume, is the Total capacity.

Different backend storage systems can thin-provision BlockVolumes.
Initially the Used storage will be low, and growing over time. This is
not detectable through standard Operating System APIs, but the backend
storage system can provide more acurate Available/Used stats through
expert APIs.

In order to allow the Node to retrieve advanced stats from the backend
storage system, the secrets that are used for staging the volume can be
provided.

csi.proto

@@ -1395,6 +1395,13 @@ message NodeGetVolumeStatsRequest {
   // system/filesystem, but, at a minimum, SP MUST accept a max path
   // length of at least 128 bytes.
   string staging_target_path = 3;
+
+  // Secrets that the plugin can use to request additional statistics
+  // from the backend storage system.
+  // This field is OPTIONAL. Refer to the `Secrets Requirements`
+  // section on how to use this field.
+  map<string, string> secrets = 4
+    [(csi_secret) = true, (alpha_field) = true];
 }
 
 message NodeGetVolumeStatsResponse {