Remove metrics global registry

I would like to suggest to remove the global registry as currently the controller-runtime global registry does not capture metrics registered onto `prometheus.DefaultRegisterer`. As a step forward and in line with the prosed [KEP](https://github.com/kubernetes/community/pull/2909), the controller-runtime should be able to have the registry injected. (for a start with `prometheus.DefaultRegisterer`, with the long term goal of removing the global registry in the first place, I am open for suggestions on that.)

As per the proposal https://github.com/kubernetes/community/pull/2909:

> Kubernetes also makes extensive use of a global metrics registry to register metrics to be exposed. Aside from general shortcomings of global variables, Kubernetes is seeing actual effects of this, causing a number of components to use sync.Once or other mechanisms to ensure to not panic, when registering metrics. Instead a metrics registry should be passed to each component in order to explicitly register metrics instead of through init methods or other global, non-obvious executions.

Lastly, this may be my own opinion, but introducing globals as a library hides its dependencies, therefore I think having the registry be injected makes the API more clear.

The issue names no files, tests, or entry points. Start by reviewing the controller-runtime metrics implementation and the linked Kubernetes KEP, then define the scope for injecting a Prometheus registry and what removal of the global registry would require.

Written by the indexing model from the issue text.