Add external Prometheus address setting to gitlab.yml

What does this MR do?

This MR:

• For user configuration: add external Prometheus listen address(gitlab_rails['prometheus_server_address']) in gitlab.rb.template
• For generated gitlab.yml, add a new setting Prometheus.server_address. Existing Prometheus.enable and Prometheus.listen_address will respect Prometheus.server_address as well.

So that:

• user can set external Prometheus listen address in /etc/gitlab/gitlab.rb
• the value can be accessed in the Rails app and Sidekiq job.

Related issues

gitlab#30175 (closed)

Todo

Update documentations: https://docs.gitlab.com/ee/administration/monitoring/prometheus/, and https://docs.gitlab.com/ee/administration/high_availability/monitoring_node.html.

Checklist

See Definition of done.

• Changelog entry created. Not applicable for Documentation changes and minor changes.
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA, if applicable
• MR targeting master branch
• MR has a green pipeline on GitLab.com
• Equivalent MR/issue for CNG opened if applicable
• trigger-package has a green pipeline running against latest commit

Reviewer Checklist

In addition to above, reviewer must:

• Pipeline is green on dev.gitlab.org if the change is not touching documentation or internal cookbooks