Remove unneeded rake task to seed metrics dashboard
What does this MR do and why?
Changes: Removes the metrics:setup_common_metrics rake task and docs related to populating the database with metrics for the metrics dashboard.
Context: This MR cleans up a portion of dead code related to the removal of devopsmonitor Category:Metrics features. The Metrics Dashboard & Prometheus Metrics/Alerts (used for monitoring apps deploying using gitlab) were deprecated in 14.7 and removed from the product in 16.0.
- grouprespond related issue: Metrics: Delete models & database tables/columns (#397142)
- Monitor:Metrics removal epic: Monitor: Metrics Deprecation and Removal (&10107)
- Reference MR with all related removals: Draft: Remove all remaining metrics dashboard-r... (!160325 - closed)
- Metrics removal is blocking cells: Drop old unused tables for prometheus alerts (#461838 - closed)
MR acceptance checklist
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How to set up and validate locally
These features were disabled in %16.0 and the code has been partially deleted since then. So there's nothing really to test here. It's no longer possible to set up these features.