void fence(int brokerId) {
        BrokerHeartbeatState broker = brokers.get(brokerId);
        if (broker != null) {
            untrack(broker);
        }
    }
Mark a broker as fenced.

@param brokerId      The ID of the broker to mark as fenced.
metadata/src/main/java/org/apache/kafka/controller/BrokerHeartbeatManager.java