docs: public void addProducer(Broker broker) {
          Properties props = new Properties();
          props.put("host", broker.host);
          props.put("port", "" + broker.port);
          props.putAll(config.getProperties());
          if (sync) {
              SyncProducer producer = new SyncProducer(new SyncProducerConfig(props));
              logger.info("Creating sync producer for broker id = " + broker.id + " at " + broker.host + ":" + broker.port);
              syncProducers.put(broker.id, producer);
          } else {
              AsyncProducer<V> producer = new AsyncProducer<V>(new AsyncProducerConfig(props),//
                      new SyncProducer(new SyncProducerConfig(props)),//
                      serializer,//
                      eventHandler,//
                      config.getEventHandlerProperties(),//
                      this.callbackHandler, //
                      config.getCbkHandlerProperties());
              producer.start();
              logger.info("Creating async producer for broker id = " + broker.id + " at " + broker.host + ":" + broker.port);
              asyncProducers.put(broker.id, producer);
          }
      }