boolean arePartitionLeadersImbalanced() {
        return !imbalancedPartitions.isEmpty();
    }
Process a broker heartbeat which has been sitting on the queue for too long, and has
expired. With default settings, this would happen after 1 second. We process expired
heartbeats by updating the lastSeenNs of the broker, so that the broker won't get fenced
incorrectly. However, we don't perform any state changes that we normally would, such as
unfencing a fenced broker, etc.
metadata/src/main/java/org/apache/kafka/controller/ReplicationControlManager.java