code: def add_broker(self, broker):
          
          if broker not in self._brokers:
              self._brokers.add(broker)
          else:
              self.log.warning(
                  'Broker {broker_id} already present in '
                  'replication-group {rg_id}'.format(
                      broker_id=broker.id,
                      rg_id=self._id,
                  )
              )
docs: Add broker to current broker-list.