code: def _compute_metadata_from_readers(self):
          
          mda = {'sensor': self._get_sensor_names()}
  
          # overwrite the request start/end times with actual loaded data limits
          if self.readers:
              mda['start_time'] = min(x.start_time
                                      for x in self.readers.values())
              mda['end_time'] = max(x.end_time
                                    for x in self.readers.values())
          return mda
docs: Determine pieces of metadata from the readers loaded.