✨ Allow users to create custom registries

[sonamkshenoy]

This PR addresses #2670

Currently, when users import the Registry that the controller-runtime provides, they get a whole lot of metrics which might not be relevant to them.
This PR aims at allowing users to pass a custom registry of their own and register only those metrics that are relevant.

Thus if a user needs only the work queue and controller metrics, they just need to do:

customRegistry := prometheus.NewRegistry()
metrics.RegisterWorkqueueMetrics(customRegistry)
metrics.RegisterReconciliationMetrics(customRegistry)

[k8s-ci-robot]

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This pull-request has been approved by: sonamkshenoy
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