Add metrics dashboard template finder

What does this MR do?

This adds the necessary backend code that handles the metrics dashboard templates, all of the dashboard templates will be under the *.metrics-dashboard.yml suffix. The logic behind the templates follows the same conventions as the .gitlab-ci.yml and the Dockerfile templates

#220178 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content