@Override
    public void gracefulShutdown() {
        LOG.info("Starting a graceful shutdown of {}", clientConfig.getClientName());
        shuttingDown = true;
        dynamicServerResolver.shutdown();
        perServerPools.values().forEach(IConnectionPool::drain);
    }
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