Supply volume fs metrics to server/stats/handler.go

* Metrics will not be expose until they are hooked up to a handler
* Metrics are not cached and expose a dos vector, this must be fixed before release or the stats should not be exposed through an api endpoint

hack/verify-flags/known-flags.txt

@@ -365,6 +365,7 @@ use-kubernetes-cluster-service
 user-whitelist
 verify-only
 volume-plugin-dir
+volume-stats-agg-period
 watch-cache
 watch-only
 whitelist-override-label