code: def add_response_log_fields(self, log_fields: LogFields,
                                  start_time: datetime, err: Exception):
          
          code = "Unknown" if err is not None else "OK"
          duration = (datetime.utcnow() - start_time).total_seconds() * 1000
          log_fields.add_fields({
              "grpc.start_time": start_time.isoformat() + "Z",
              "grpc.code":       code,
              "duration":        "{duration}ms".format(duration=duration),
          })
docs: Add log fields related to a response to the provided log fields
  
          :param log_fields: log fields instnace to which to add the fields
          :param start_time: start time of the request
          :param err: exception raised during the handling of the request.