@Override
    public Promise<PooledConnection> acquire(EventLoop eventLoop) {
        return acquire(eventLoop, null, CurrentPassport.create(), new AtomicReference<>(), new AtomicReference<>());
    }
Gracefully shuts down a DefaultClientChannelManager by allowing in-flight requests to finish before closing the connections.
Idle connections in the connection pools are closed, and any connections associated with an in-flight request
will be closed upon trying to return the connection to the pool
zuul-core/src/main/java/com/netflix/zuul/netty/connectionpool/DefaultClientChannelManager.java