public int size() {
        final Lock lock = this.lock;
        lock.lock();
        try {
            return this.queue.size();
        }
        finally {
            lock.unlock();
        }
    }
A request is not referenced by this queue,so we don't want to add
it.
metamorphosis-client/src/main/java/com/taobao/metamorphosis/client/consumer/FetchRequestQueue.java