Add Metrics Dashboard GraphQL alert queries to FF

What does this MR do and why?

This MR gates the metrics dashboard graphql alert queries behind the remove_monitor_metrics feature flag. The frontend feature that used this field was actually removed a very long time ago. But we also want to deprecate/remove any API access.

Related issue: Metrics: Delete API entrypoints & helpers (#397138 - closed) (one of many MRs)

Context for removal:

• The Metrics Dashboard feature is being removed in %16.0.
  • Feature docs: https://docs.gitlab.com/ee/operations/metrics
  • Removal issue: Monitor: Metrics Deprecation and Removal (&10107)
• Most of the metrics dashboard frontend is currently behind the remove_monitor_metrics flag, but the flag has not yet been enabled. It needs to be enabled by default in time for %16.0.
  • The entire frontend should be behind the flag in order to enable it by default, but the API can merge after - as long as we make all breaking changes within %16.0.
  • Rollout issue: [Feature flag] Rollout of `remove_monitor_metrics` (#399248)
  • Tracking issue: Metrics: hide deprecated modules behind a featu... (#399231 - closed)
  • Enable-by-default MR: Set remove_monitor_metrics flag to true (!119989 - merged)

All related backend MRs to flag all metrics dashboard endpoints

• Add Metrics Dashboard annnotations API to FF (!120286 - merged)
• Add Metrics Dashboard starred dashboard API to FF (!120288 - merged)
• Add Metrics Dashboard templates viewer to FF (!120289 - merged)
• Add Metrics Dashboard environments API to FF (!120299 - merged)
• Add Metrics Dashboard settings API to FF (!120305 - merged)
• Add Metrics Dashboard GraphQL alert queries to FF (!120307 - merged)
• Add Metrics Dashboard embed pipeline to FF (!120310 - merged)
• Add Metrics Dashboard for internal dashboard en... (!120313 - merged)
• Add Metrics Dashboard base API to FF (!120316 - merged)

How to set up and validate locally

** pre-req: project with an environment

1. Create a prometheus alert with the right attributes via the rails console

   environment = Environment.last
   project = environment.project
   payload = JSON.parse({
     "labels": {
       "gitlab_environment_name": environment.name
     },
     "status": "firing",
     "startsAt": "2021-03-17T11:22:40Z",
     "annotations": {
       "title": "Testing that prometheus alerts do not re-acknowledge",
       "gitlab_y_label": "status"
     },
     "generatorURL": "http://host?g0.expr=up"
   }.to_json)
   
   AlertManagement::ProcessPrometheusAlertService.new(project, payload).execute

2. Use http://gdk.test:3000/-/graphql-explorer to query for alert details

Query	Response before	Response after
query alerts { project(fullPath: "namespace1/project-1") { alertManagementAlerts { nodes { id title metricsDashboardUrl } } } }	{ "data": { "project": { "alertManagementAlerts": { "nodes": [ { "id": "gid://gitlab/AlertManagement::Alert/7", "title": "Testing that prometheus alerts do not re-acknowledge", "metricsDashboardUrl": "http://gdk.test:3000/namespace1/project-1/-/environments/1/metrics_dashboard?embed_json=%7B%22panel_groups%22%3A%5B%7B%22panels%22%3A%5B%7B%22type%22%3A%22area-chart%22%2C%22title%22%3A%22Testing+that+prometheus+alerts+do+not+re-acknowledge%22%2C%22y_label%22%3A%22status%22%2C%22metrics%22%3A%5B%7B%22query_range%22%3A%22up%22%7D%5D%7D%5D%7D%5D%7D&embedded=true&end=2021-03-17T11%3A52%3A40Z&start=2021-03-17T10%3A52%3A40Z" } ] } } } }	{ "data": { "project": { "alertManagementAlerts": { "nodes": [ { "id": "gid://gitlab/AlertManagement::Alert/7", "title": "Testing that prometheus alerts do not re-acknowledge", "metricsDashboardUrl": null } ] } } } }

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

app/graphql/types/alert_management/alert_type.rb

@@ -115,7 +115,7 @@ class AlertType < BaseObject
             GraphQL::Types::String,
             null: true,
             description: 'URL for metrics embed for the alert.',
-            deprecated: { reason: 'Returns no data. Feature was deprecated in 14.7 & removed in 16.0',
+            deprecated: { reason: 'Returns no data. Underlying feature was removed in 16.0',
                           milestone: '16.0' }
 
       field :runbook,