MINOR: Log pending join members

[dajac]

I am still chasing KAFKA-17493. I was able to narrow it down to an issue with the pending join members. This patch logs them in order to help me troubleshooting it further. I will revert this change when the issue is root caused.

Committer Checklist (excluded from commit message)

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

[mumrah]

If we need to ensure we revert this eventually, and assuming it's actually too verbose to ship with, can you file a blocker ticket for 4.0 just so we don't lose track?

Another approach would be to do a separate trace log with the extra info.

if(log.isTraceEnable()) { 
  log.trace(...) 
} else { 
  log.debug(...) 
}

Either way LGTM. Happy bug hunting :)

[dajac]

Sure thing: https://issues.apache.org/jira/browse/KAFKA-17571.