Adds Labkit::Metrics behind an environment variable

What does this MR do and why?

Part of Replace prometheus client in the Rails app (mon... (gitlab-com/gl-infra/observability/team#4160 - closed).

Depends on Adding the Metrics module (gitlab-org/ruby/gems/labkit-ruby!135 - merged).

Adds Labkit::Metrics behind an environment variable.

• Pipeline run with LABKIT_METRICS_ENABLED: "true”: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1855538344
• Pipeline run without LABKIT_METRICS_ENABLED set: https://gitlab.com/gitlab-org/gitlab/-/pipelines/1857261519

The coverage pipeline is failing (as expected):

• https://gitlab.com/gitlab-org/gitlab/-/jobs/10277200332
• https://gitlab.com/gitlab-org/gitlab/-/jobs/10279133990

In one case, with LABKIT_METRICS_ENABLED=true, it complains that Gitlab::Metrics::Prometheus isn't hit, which is correct, since only Gitlab::Metrics::Labkit will be used. And vice-versa.

The change request to toggle the environment variable in gstg is available here.

A follow-up MR will get rid of Gitlab::Metrics::Prometheus, as part of Replace prometheus client in the Rails app (mon... (gitlab-com/gl-infra/observability/team#4160 - closed).

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. export LABKIT_METRICS_ENABLED=true
2. gdk start
3. Visit http://127.0.0.1/-/metrics

MR acceptance checklist

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