docs: def usage(self, auth, resource, metric, starttime, endtime, defer=False):
          """ Returns metric usage for client and its subhierarchy.
  
          Args:
              auth: <cik> for authentication
              resource: ResourceID
              metrics: Metric to measure (as string), it may be an entity or consumable.
              starttime: Start time of window to measure useage (format is ___).
              endtime: End time of window to measure useage (format is ___).
          """
          return self._call('usage', auth,
                            [resource, metric, starttime, endtime], defer)