boolean checkOverload() {
        // If all unfenced brokers are stale, the controller is in overload state.
        BrokerHeartbeatState last = unfenced.last();
        if (last != null && !hasValidSession(last.id)) {
            // all brokers are stale
            overloadCircuitBreaker.overload();
            return true;
        }
        return false;
    }
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