This adds logic to hook into two other metric systems:
1. `cache.SetReflectorMetricsProvider`, which doesn't seem hooked up in Kubernetes yet, but would theoretically give us metrics about the mechanisms underpinning informers.
2. `metrics.Register`, which hooks us into the rest client infrastructure to give us metrics about low-level API server calls.
Fixes: https://github.com/knative/pkg/issues/679
Fixes: https://github.com/knative/pkg/issues/680
Matt Moore