very poor choice of logging if client fails to connect to server

We are using ZooKeeper in our project and have received reports that, when suffering a networking problem, log files become flooded with messages like:

07 Sep 2017 08:22:00 (System) [] Session 0x45d3151be3600a9 for server null, unexpected error, closing socket connection and attempting reconnect
java.net.NoRouteToHostException: No route to host
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method) ~[na:1.8.0_131]
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:717) ~[na:1.8.0_131]
at org.apache.zookeeper.ClientCnxnSocketNIO.doTransport(ClientCnxnSocketNIO.java:361) ~[zookeeper-3.4.6.jar:3.4.6-1569965]
at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1081) ~[zookeeper-3.4.6.jar:3.4.6-1569965]

Looking at the code that logs this message (ClientCnxn), there seems to be quite a few problems here:

1. the code logs a stack-trace, even though there is no bug here. In our project, we treat all logged stack-traces as bugs,
2. if the networking issue is not fixed promptly, the log files is flooded with these message,
3. The message is built using ClientCnxnSocket#getRemoteSocketAddress, yet in this case, this does not provide the expected information (yielding null),
4. The log message fails to include a description of what actually went wrong.

(Additionally, the code uses string concatenation rather than templating when building the message; however, this is an optimisation issue)

My suggestion is that this log entry is updated so that it doesn't log a stack-trace, but does include some indication why the connection failed.