- [ ] 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.
handler.sent(channel, msg);
                    return;
                }
                Throwable t = future.cause();
                if (t != null && isRequest) {
                    Request request = (Request) msg;
                    Response response = buildErrorResponse(request, t);
                    handler.received(channel, response);
                }
            } finally {
                NettyChannel.removeChannelIfDisconnected(ctx.channel());
            }
        });
    }
    @Override
    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
    	
        if (evt instanceof IdleStateEvent) {
            try {
                NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
                if (logger.isDebugEnabled()) {
                    logger.debug("IdleStateEvent triggered, send heartbeat to channel " + channel);
                }
                Request req = new Request();
                req.setVersion(Version.getProtocolVersion());
                req.setTwoWay(true);
                req.setEvent(Request.HEARTBEAT_EVENT);
                channel.send(req);
            } finally {
                NettyChannel.removeChannelIfDisconnected(ctx.channel());