docs: def _RegisterFlowProcessingHandler(self, handler):
      """Registers a handler to receive flow processing messages."""
      self.flow_handler_stop = False
      self.flow_handler_thread = threading.Thread(
          name="flow_processing_handler",
          target=self._HandleFlowProcessingRequestLoop,
          args=(handler,))
      self.flow_handler_thread.daemon = True
      self.flow_handler_thread.start()