Refactor Prometheus usage to explicitly pass a registry

[tstirrat15]

We're currently implicitly using the default registry, which means that tests sometimes flake because two different tests attempt to add the same metric to the same registry, and there's functionality where you can explicitly set up and pass a registry: https://pkg.go.dev/github.com/prometheus/client_golang/prometheus#hdr-Advanced_Uses_of_the_Registry

If we moved to that approach, we could have each test provide its own registry and avoid duplicate registration.

[tstirrat15]

This may mean we can't use promauto. Hopefully that isn't too big an issue.