PartitionsOnReplicaIterator partitionsLedByBroker(int brokerId) {
        return iterator(brokerId, true);
    }
Update our records of a partition's ISR.

@param topicId       The topic ID of the partition.
@param partitionId   The partition ID of the partition.
@param prevIsr       The previous ISR, or null if the partition is new.
@param nextIsr       The new ISR, or null if the partition is being removed.
@param prevLeader    The previous leader, or NO_LEADER if the partition had no leader.
@param nextLeader    The new leader, or NO_LEADER if the partition now has no leader.
metadata/src/main/java/org/apache/kafka/controller/BrokersToIsrs.java