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