Optional<Integer> findOneStaleBroker() {
        // If the controller is in overload state, we should not fence any broker.
        if (overloadCircuitBreaker.isOverload()) {
            return Optional.empty();
        }
        Optional<Integer> staleBroker = findOneStaleBroker0();
        if (staleBroker.isEmpty()) {
            return staleBroker;
        }
        // Check whether the controller is in overload state.
        if (checkOverload()) {
            return Optional.empty();
        } else {
            return staleBroker;
        }
    }
Check if the oldest broker to have heartbeated has already violated the
sessionTimeoutNs timeout and needs to be fenced.

@return      An Optional broker node id.
metadata/src/main/java/org/apache/kafka/controller/BrokerHeartbeatManager.java