Support custom metrics provider for controller_runtime metrics

[jonathan-innis]

We have use-cases for using different metric providers for metrics that are provided by controller runtime (e.g. printing EMF logs rather than using a Prometheus client). Currently, there is no way to register a custom metrics provider with controller runtime like is allowed through metrics like client-go.

Metrics are registered here but are statically configured to use Prometheus as their metric publisher. It would be great if we could support a similar provider registration mechanism like we have with client-go.