- [ ] 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.
public class NettyServerHandler extends ChannelDuplexHandler {
    private static final Logger logger = LoggerFactory.getLogger(NettyServerHandler.class);
    /**
     * the cache for alive worker channel.
     * <ip:port, dubbo channel>
     */
    private final Map<String, Channel> channels = new ConcurrentHashMap<String, Channel>();
    private final URL url;
    private final ChannelHandler handler;
    public NettyServerHandler(URL url, ChannelHandler handler) {
        if (url == null) {
            throw new IllegalArgumentException("url == null");
        }
        if (handler == null) {
            throw new IllegalArgumentException("handler == null");
        }
        this.url = url;
        this.handler = handler;
    }
    public Map<String, Channel> getChannels() {
        return channels;
    }
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
        try {
            if (channel != null) {
                channels.put(NetUtils.toAddressString((InetSocketAddress) ctx.channel().remoteAddress()), channel);
            }