Add Prometheus-backed metrics to HPAs (if configured)
Add Prometheus-backed metrics to HPAs (if configured)

We want to use both the SFX and Prometheus adapters for now, so we'll
add two different metric specs to our HPAs so that both of the adapters
are used concurrently (and, if there are issues with one we don't
completely kill all autoscaling)

Yelp/paasta,Yelp/paasta