✨ allow user to specify metrics

[mengqiy]

fixes #258

The tests are failing because currently different test cases try to register same metrics in the shared Registry. I will fix tests if we decide to proceed with this approach.

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[DirectXMan12]

This case made me a bit nervous when we first introduced this. I'm not sure this is the right way to solve it, though...

[DirectXMan12]

/hold

[terinjokes]

@DirectXMan12 @mengqiy Is there something I can help with from a user use-case?

I understand the desire elsewhere to make it really easy to have working metrics, but registering default metrics, even if the user provides their own collectors, makes it extremely difficult to use controller-runtime in a larger project.

[DirectXMan12]

@terinjokes see my comment on #258 for my comments on the buckets. For the default collectors, go runtime metrics, etc we might just want to disable them. Which are you taking about?

[mengqiy]

Closing this until we decided how we tackle it.