Tests for Prometheus alerting rules

Description

Thanks to the prometheus-alerts project, CI test jobs were implemented for Prometheus (!90 (merged)).

But configs/prometheus/test.yml still lacks proper testing.

Tasks

• Add input series on test.yml.
• Implement initial tests for the following situations:
  • Onion Service descriptor unreachable.
  • Invalid certificate for the Onion Service.
  • Expiring certificate for the Onion Service.
  • Onion Service unreachable.
  • Invalid status code.
• Improvements:
  • Add the quick HOWTO into the development documentation. Done with fae72903. Done in !103 (merged).
  • Consider additional series and tests for the situations above.
  • Consider to improve the alerts metadata to be more informative and helpful to address this feedback. Done on !102 (merged).
  • ChangeLog update.
  • Inform downstream about the improvements (thread). Done.