WIP: Connect via Kubernetes to Prometheus running in Kubernetes cluster

What does this MR do?

Interacts with Prometheus API via Kubernetes proxy and using Kubernetes Service.

This is working prototype.

This expects to have prometheus service to be running in prometheus namespace, exposed on port 9090.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/30480