EE Port: gitlab-ce!29186 Update server API to enable switch prometheus endpoint
What does this MR do?
EE port of gitlab-ce!29186
Backendian parts of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27849
- Make no_content response work with frontend polling (expects 204)
- Expose a complete prometheus endpoint in conjunction with metrics dashboard. The dashboard goes through a selection of processing stages. This adds a new stage which inserts the api path.
Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/58516