Extract Metrics Prometheus API proxy logic into shared module

What does this MR do?

This is next step in backend efforts towards #208224 (closed) (moving cluster health to the core). Because logic which proxied requests to Prometheus API was duplicated in PrometheusApiController and ClustersController instead of moving code from ee/app/controllers/ee/clusters/clusters_controller.rb to app/controllers/clusters/clusters_controller.rb I've decided to move it into stand alone module which can be included in both mentioned controllers, that way moving cluster health from ee can be achieved along with removing duplicated code. I've also extracted shared test examples, to be reused in each of controller specs. !35610 (merged) is a follow up MR which actually moves code out of ee

Does this MR meet the acceptance criteria?

Conformity

~backstage change

- [ ] Changelog entry

- [ ] Documentation (if required)

• Code review guidelines
• Style guides
• Separation of EE specific content