✨ Define metrics.Registry as an interface

[xrstf]

It's currently not possible to replace the predefined Registry with a custom implementation.

I need this because I re-create controllers with the same name at runtime and this causes the default registry to rightfully complain that the metrics for this label combination are already registered. When calling controller.New() I have no influence over the metrics creation and errors are always logged with a logger I can also not control.

With this PR I can register my own Registry implementation that silently drops the AlreadyRegistered errors.

We could have used a custom subset of the two predefined Registerer/Gatherer interfaces, but both are so short and only contain what we actually need, so it seemed not worth the effort to replicate the method definitions.

I can see that there are some long-standing intentions to migrate to both non-global metrics definitions and the use of OpenTelemetry, but in the meantime this would make life easier [for me]. :-)

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[alvaroaleman]

/lgtm
/ok-to-test
/assign @DirectXMan12

[alvaroaleman]

/lgtm

[DirectXMan12]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: DirectXMan12, xrstf

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[DirectXMan12]

/retest

o_O looks like a flake? (asked the author of the PR that added that test to take a look)