Add cron for prometheus integration health check

What does this MR do and why?

Adds a cron definition for the prometheus integration health check.

See #322581 (closed)

Screenshots or screen recordings

Using the steps below (I changed the minute to 16 locally):

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Change the 0 in the cron definition to an upcoming minute
2. Start GDK
3. From GDK root, grep CheckPrometheus gitlab/log/sidekiq.log
4. Observe the job running and succeeding

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.