Added Prometheus Service and Prometheus graphs

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/26910

This MR adds a Prometheus Service, with support for fetching metrics for an environment and displaying that on environments page.

We miss actual data fetching, but this is a placeholder that can be used to be worked on.

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
• 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?

Functionality Notes

Kamil: Please, edit queries and @jivanvl can add graphs to metrics action of environments_controller, as part of my MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8935

Kamil: For development make sure to have sidekiq running as the data is fetched in the background, API can return the http status code 204 it means that the data is not yet ready, retry with some delay if that happens. In normal circumstances you will receive a 200 http status code with the JSON status: true

Josh: I spun up a prometheus server outside of the omnibus package, here: https://kubeprom.35.185.3.210.nip.io/graph. This one has the k8s metrics we want to use for this.

\cc @joshlambert @ayufan

lib/api/v3/services.rb

@@ -423,6 +423,14 @@ class Services < Grape::API
             desc: 'Comma-separated list of branches which will be automatically inspected. Leave blank to include all branches.'
           }
         ],
+        'prometheus' => [
+          {
+            required: true,
+            name: :api_url,
+            type: String,
+            desc: 'Prometheus API Base URL, like http://prometheus.example.com/'
+          }
+        ],
         'pushover' => [
           {
             required: true,