Expose a /metrics endpoint in the devfile registry

[johnmcollier]

As part of the process to deploy a public devfile registry on OpenShift Dedicated, we need to make sure there is a Prometheus /metrics endpoint exposed in both the registry index server and OCI registry to be scraped.

https://prometheus.io/docs/guides/go-application/ provides some straightforward instructions on setting up a metrics endpoint in a Go webserver. We should also look at what additional metrics we would want to provide that are relevant / useful regarding the state of the Devfile Registry.

• Add the prometheus Go module to the index server, and set up a /metrics endpoint using the HTTP handler provided by the package
• Enable Prometheus on the OCI registry server
• Determine what additional metrics that we should provide, in addition to what Prometheus already provides for us

[johnmcollier]

PR for metrics on the OCI Registry: devfile/registry-operator#8
PR for metrics on the index server: devfile/registry-support#19