Remove Deprecated Prometheus - Use OpenCensus Instead about gizmo

[jprobinson]

I'm in for hooking Prometheus into the kit server as well. Following along with my comments in the gcputil branch, I wonder if we can just have that package return the exporters we need to register.

Internally, I wonder if that new package could be something like observe.NewOCExporter(...) and it would scan the environment (or some simple config?) to determine which exporters to initiate and respond with them. This could check for Prom configs, init that if they exist then fall back to checking GCP creds/Stackdriver options.

I like the idea of (3) too, though: Allow users to pass a custom exporter on their own. I just wonder if thats a use case we need.