docs: def _get_metadata(self, file_id, metadata_fields=''):
          
          ''' a helper method for retrieving the metadata of a file '''
          
          title = '%s._get_metadata' % self.__class__.__name__
          
      # construct fields arg
          if not metadata_fields:
              metadata_fields = ','.join(self.object_file.keys())
          else:
              field_list = metadata_fields.split(',')
              for field in field_list:
                  if not field in self.object_file.keys():
                      raise ValueError('%s(metadata_fields="%s") is not a valid drive file field' % (title, field))
      
      # send request
          try:
              metadata_details = self.drive.get(fileId=file_id, fields=metadata_fields).execute()
          except:
              raise DriveConnectionError(title)
          
          return metadata_details