Updating a self hosted GitLab Omnibus instance from 11.0.2 to 11.3.4 on a CentOS 7.4 for me resulted in the bundled Prometheus using almost 100% of available CPU time. Naturally all other GitLab operation is impacted by this and excessively slow.
So far I was not able to pinpoint what the problem is.
I could need advice on how to properly diagnose and fix this issue.
Thanks for pointing me in the right direction.