Prometheus metrics

[JoelSpeed]

I would like to add prometheus metrics to the controllers I'm building using kubebuilder.

It would be good to have information both from the controller internals themselves as well as from the reconciler loops I'm implementing.

My initial idea is to add a prometheus metric registry to the controller manager that it can use itself and can be used by reconcil.Reconcilers to register their own metrics. Then have the controller manager start the metrics server when Start is called.

Does this seem reasonable or should people just handle metrics on their own? PR welcome?

[DirectXMan12]

PRs definitely welcome. We already have some metrics that we inherit from the underlying apimachinery, but PRs are definitely welcome to improve this!

/kind feature