Document how and when to run Puma in Single mode, limitations

• We need to document how to run Puma in Single mode
• We need to tell when it's relevant (for memory contained env)
• We need to explain the limitations (and link the issues from this epic)

A wider issue on running in limited env Documentation: #289909 (closed)

How to start Puma in Single mode

• In /etc/gitlab/gitlab.rb, set puma['worker_processes'] = 0 (see https://github.com/puma/puma/blob/5af91ff6aa14ab373184ad908d5ff5ac637eda4a/lib/puma/launcher.rb#L367)
• Set PUMA_SKIP_CLUSTER_VALIDATION to true in gitlab.rb (!51906 (merged))
• sudo gitlab-ctl reconfigure

Note on Limitations:

• Metrics - #300652 (closed) - will be fixed
• Health Checks are broken - #300661 (closed) (not in work yet)
• Graceful shutdown will not work - should we consider this as an issue and try to fix it?
• Memory Killer is no longer applicable - this seems logical. https://github.com/schneems/puma_worker_killer - docs. I don't think we could find a good workaround, as we will need to restart a full app, which will result in a downtime.