Prometheus integration for registry-v2

[bcpeinhardt]

We need to setup a Prometheus integration for registry-v2. Ideally we'll be able to use this official Prometheus client to get all the information we need. If not, we may need to throw nginx or some other web server in front of the registry as a reverse proxy so we can use a Prometheus exporter instead.

As far as metrics we want to collect, we basically want logs of requests to the service (including responses) and some basic performance metrics like memory usage and response times.

Along with this, we want to set up alerting on unexpected issues (500's basically). We use PagerDuty for alerting.

Once we have this set up, we'll want to configure our existing Prometheus instance to scrape these metrics.