Title: New prometheus USE flag 
Author: Horst Prote <prote@fmi.uni-stuttgart.de>
Posted: 2021-12-22
Revision: 1
News-Item-Format: 2.0

In /opt/gitlab/gitlab/doc/administration/monitoring/prometheus/index.md one reads
  Prometheus is a powerful time-series monitoring service, providing a flexible
  platform for monitoring GitLab and other software products.

  GitLab provides out-of-the-box monitoring with Prometheus, providing easy
  access to high quality time-series monitoring of GitLab services.

  Prometheus and the various exporters listed in this page are bundled in the
  Omnibus GitLab package. [...] For installations from source you must install
  them yourself.

With the latest changes in version 14.6.0 the startup of the sidekiq service
fails if there is no Prometheus metrics server running. One either has to
- install, setup and run a Prometheus server (I think so; haven't tried it)
- or disable "sidekiq_exporter:" in the gitlab.yml file.

So I added the new prometheus USE flag, which is off by default and then
disables "sidekiq_exporter:", "web_exporter:" and "prometheus:" in gitlab.yml.