Coverage in CI is sometimes flaking. It seems this is caused by a single line sometimes not getting hit (the coverage threshold is very tight). Sample CI runs to illustrate:

https://12132-180498819-gh.circle-artifacts.com/0/coverage/coverage/proc/self/cwd/source/client/client_worker_impl.cc.html#L55

https://12111-180498819-gh.circle-artifacts.com/0/coverage/coverage/proc/self/cwd/source/client/client_worker_impl.cc.html#L55

I had a quick look, and as far as I can tell, client_worker_test ought to always cover this line / I can't see a way that it wouldn't do so.