Add a prometheus proxy API per environment

What does this MR do?

• Wait for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/26840 hence the WIP

Add an API to proxy requests to an environment's Prometheus server.

EE port: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10530

What are the relevant issue numbers?

#58375 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #58375 (closed)