[RUN AS-IF-FOSS] Move Promethues API proxy for cluster health into the core

What does this MR do?

This MR is next step towards moving cluster health into the core (#208224 (closed)) It moves logic which allows cluster health dashboard to make request to Prometheus API (data source for cluster health dashboard) via GitLab proxy endpoint.

Does this MR meet the acceptance criteria?

Conformity

I still consider this a ~backstage change because until #metrics_dashboard will be moved into the core, no change can be perceived from user perspective. Therefore I've not included changes to documentation nor changelog note yet.

- [ ] Changelog entry

- [ ] Documentation (if required)

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content