- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.3
* Operating System version: macOs sierra
* Java version: 1.8

### Steps to reproduce this issue

Sometimes, when debugging connect time issue, we don't know whether the port is listening.  We can check the connected log if the connect timeout occurs.
} finally {
            NettyChannel.removeChannelIfDisconnected(ctx.channel());
        }
    }
    @Override
    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
        
        if (evt instanceof IdleStateEvent) {
            NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
            try {
                logger.info("IdleStateEvent triggered, close channel " + channel);
                channel.close();
            } finally {
                NettyChannel.removeChannelIfDisconnected(ctx.channel());
            }
        }
        super.userEventTriggered(ctx, evt);
    }
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
            throws Exception {
        NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
        try {
            handler.caught(channel, cause);
        } finally {
            NettyChannel.removeChannelIfDisconnected(ctx.channel());
        }
    }
}