MINOR: add logs when connection count exceeds configured count

[showuon]

Currently, when client tried to connect to broker, and got throttled or exceeding connection count (waiting for available connection slot), there will be no logs output in broker. All the client can see is connection timeout exception. This PR adds logs for this case.

Committer Checklist (excluded from commit message)

• [ ] Verify design and implementation
• [ ] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)

[showuon]

@mimaison @rajinisivaram , please take a look. Thanks.

[dajac]

I am not sure about it. In highly loaded system with a lot of connections, this will result in flooding the logs. This is even worst if there are rogue clients. It would be better to rely on metrics to diagnose such issues. I don't remember, do we have metrics for this?

[showuon]

@dajac, thanks for the comment. Yes, I agree with you that this could result in flooding the logs in highly loaded system. But still, if the client got blocked due to exceeding max connection limit, it's bad that there's no log in broker side. Expose the info via metrics is a good suggestion. At quick look, I can't find we have any metrics recording this info. I'll check it again later, if no, I'm going to propose a KIP for it.

But back to this PR, it is still not good when this issue happened, but broker keeps silent, even if we have metric's help. Maybe we can log with DEBUG level like @divijvaidya 's suggestion, and user can see this log when troubleshooting issues. WDYT?