public void onBrokerChange(String topic, BrokerChangeListener listener) {
        CopyOnWriteArraySet<BrokerChangeListener> set = this.getListenerList(topic);
        set.add(listener);
    }
called when broker list changed.

@param topic
metamorphosis-client/src/main/java/com/taobao/metamorphosis/client/producer/ProducerZooKeeper.java