✨ feat: allow adding new prometheus.Gatherers on top of default registry

[jakobmoellerdev]

In case of libraries using prebuilt prometheus registries (such as controller runtime itself), it should be possible to add the gatherer to the default metrics server so that no second handler or path has to be created for them. This allows to add metrics that are precollected/registered in something outside of the own controller runtime registry

If this is rejected the only way to merge registries is to write their own custom metrics server, which seems excessive.

[k8s-ci-robot]

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This pull-request has been approved by: jakobmoellerdev
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[k8s-ci-robot]

Hi @jakobmoellerdev. Thanks for your PR.

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