Move prometheus metrics to module. Use class instance variables for metrics and metrics cache in metrics concern

/cc @bjk-gitlab
/cc @zj

```
          metric = @_metrics_provider_cache&.[](name)
          return metric if metric
``
acccessor that only slightly slower (<20ns) than direct class variable access

[CE backport] Fix intermittently failed click in dropdown in QA

See merge request gitlab-org/gitlab-ce!16738

[CE backport] Fix intermittent clone failure in QA

See merge request gitlab-org/gitlab-ce!16739

Fix avatar icon missing when Gravatar is disabled

Closes #34733 and #38715

See merge request gitlab-org/gitlab-ce!16681

Create merge request factory

See merge request gitlab-org/gitlab-ce!15964

[CE backport] Fix intermittent QA error -- Expand button not bound to JS

See merge request gitlab-org/gitlab-ce!16737

[CE backport] Fix QA new project creation click on incorrect namespace

See merge request gitlab-org/gitlab-ce!16736

Replace $.post in awards handler with axios

See merge request gitlab-org/gitlab-ce!16685

Resolve "MR "Check out branch" button should be disabled when the source branch is deleted"

Closes #42255

See merge request gitlab-org/gitlab-ce!16631

Improve Nginx configuration files

See merge request gitlab-org/gitlab-ce!16605

Mute notifications in the seeder to speed GDK development

See merge request gitlab-org/gitlab-ce!16747

Fix MR revert check when no merged_at is present

See merge request gitlab-org/gitlab-ce!16760

After source branch deletion the Merge request `Check out branch` button is disabled.

Fix JS equality rules in some specs

See merge request gitlab-org/gitlab-ce!16734

More conversions of $.ajax to axios

See merge request gitlab-org/gitlab-ce!16731

i18n flash message