Enable Prometheus monitoring of Docker Registry

Enable the Prometheus metrics endpoint on the Docker Registry debug endpoint.

• Enable /metrics on debug interface.
• Add to Prometheus scrape configs.

Closes: #3942 (closed)

	1 Warning
	You’ve made some changes at the locations which contain user facing configuration. That’s OK as long as you’re refactoring existing code and not adding any new configuration. If you are adding new user facing configuration, consider adding to gitlab.rb.template located in files/gitlab-config-template/gitlab.rb.template . Otherwise, please consider adding the ~backstage label in that case.

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@bjk-gitlab Could you also describe how to test this change? I have the necessary stuff in my prometheus and registry config files, but how do I actually test if scraping works?

@balasankarc If you connect to Prometheus (localhost:9090) you should see up{job="registry"} return 1. You can also see the metrics returned with {job="registry"}.

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Rebased against master, PTAL.

@bjk-gitlab I don't think that our version of the registry supports this. The stable version of the registry that includes https://github.com/docker/distribution/pull/2466 was just released this week 2.7.0

So this is probably blocked by #3964 (closed) for now

@twk3 You're right, it looks like 2.6.0 was branched from master about a month before this feature went in.

I will watch #3964 (closed), if that gets into 11.6, would it be OK to also bring this one in?

@bjk-gitlab I verified that the config files get generated alright. Please assign to me when #3964 (closed) is in so we can actually test it. :)

assigned to @bjk-gitlab

@balasankarc I did some testing on the current (v2.6.0) version. It ignores the prometheus config key if you add it. So it would be safe to have this in the config, even with the old version.

changed milestone to %11.7

changed milestone to %11.8

Set milestone to %11.8 since %11.7 has passed.

mentioned in issue #3653 (closed)

assigned to @twk3

The registry is updated now, this should be good to go.

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