✨Expose RegisterClientMetrics to register other clien…

[Yesphet]

…t-go metrics

Users of controller-runtime can not register custom metrics of client-go since k8s.io/client-go/tools/metrics.Register can only be called once and it's called in the init() function of pkg/metrics. So this PR expose RegisterClientMetrics for registering custom client-go metrics. If it is not called within the first 30s of startup, only the client latency metrics will be registered.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Yesphet. Thanks for your PR.

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[Yesphet]

@alvaroaleman @inteon Hi, I hope you’re doing well! Just a quick reminder to review this pr (#2936) submitted two weeks ago. Your feedback would be greatly appreciated!