Allow enabling sidekiq_exporter logs

What does this MR do?

Allow enabling sidekiq_exporter logs.

Sidekiq runs a metrics exporter process that serves Prometheus scrapes. This is a server process that emits its own logs.

These logs are disabled by default in the Rails app but can be enabled via the log_enabled switch.

This change makes this configurable in the Chart.

Related issues

gitlab-org/gitlab-metrics-exporter#19 (closed)

Test plan

• Check out this branch.
• Run:

  $ helm upgrade gitlab . \                             
    --set gitlab.sidekiq.metrics.log_enabled=true

• SSH into sidekiq pod and check that the log file exists:

  git@gitlab-sidekiq-all-in-1-v2-7d7c6bcdfd-kvjrz:/$ tail /var/log/gitlab/sidekiq_exporter.log 
  [2022-09-19T14:24:07.237+0000] INFO  WEBrick 1.6.1
  [2022-09-19T14:24:07.237+0000] INFO  ruby 2.7.5 (2021-11-24) [x86_64-linux]
  [2022-09-19T14:24:07.257+0000] INFO  WEBrick::HTTPServer#start: pid=23 port=3807
  [2022-09-19T14:24:38.487+0000] 10.12.1.19 - - [19/Sep/2022:14:24:38 UTC] "GET /metrics HTTP/1.1" 200 881 "-" "Prometheus/2.31.1"

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened. Not necessary since Omnibus already supports that.