GitLab Maintenance Mode should be exported via a Prometheus Metric

What does this MR do and why?

Adds a new worker and service class to update instance wide metrics that are not specific to any specific context (user/project/shard/vm/process)

Adds maintenance_mode state to the global metrics

How to set up and validate locally

1. Enable Sidekiq exporter (via gitlab.yml):

    sidekiq_exporter:
      enabled: true
      address: localhost
      port: 3807

2. Enable or disable maintenance mode in application settings
3. Wait for the cronjob to run (every minute)
4. Check whether gitlab_maintenance_mode metric is being exported (access http://localhost:3807/metrics):

   # HELP gitlab_maintenance_mode Multiprocess metric
   # TYPE gitlab_maintenance_mode gauge
   gitlab_maintenance_mode{pid="sidekiq_0"} 0

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #387627 (closed)