Add Prometheus Instrumentation

[sabine]

In order to measure performance characteristics and to collect better data relevant for incidents, it will be good to use Prometheus to collect relevant data points.

• [ ] instrument ocamlorg_package. See https://github.com/ocaml/ocaml.org/issues/1219.
• [ ] display some metrics on the dashboard. E.g. page rendering times, so we can more easily see where ocaml.org is slow.

Examples:

• ocaml-docs-ci adding Prometheus instrumentation: https://github.com/ocurrent/ocaml-ci/pull/507/commits/12ff6adfc2716de3d8f85c39320db49d832d36c7
• Prometheus instrumentation in ocluster: https://github.com/ocurrent/ocluster/blob/master/ocurrent-plugin/connection.ml