Assess performance of EnvironmentsController#metrics_dashboard in production

The EnvironmentsController#metrics_dashboard endpoint performs several queries & transformations on each call - synchronously and without caching the results of the DB queries. Once this endpoint is live in production, we should assess with real data whether the endpoint implementation should either perform asynchronously or further caching should be added.

Context: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27405#note_163893143 https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27405#note_162377413