Renderer and standard collectors for Prometheus monitoring

The prometheus-reporter opam package provides the parts of a metrics reporter that do not depend on a web server. Prometheus_reporter renders a registry snapshot in the Prometheus text format and registers collectors for the OCaml GC statistics.

The prometheus-reporter.unix ocamlfind library provides the Prometheus_reporter_unix module, which adds a process start-time metric and a Logs reporter that counts logged messages.