Client library for Prometheus monitoring

Applications can enable metric reporting using the prometheus-app opam package. This depends on cohttp and can serve the metrics collected above over HTTP.

The prometheus-app.unix ocamlfind library provides the Prometheus_unix module, which includes a cmdliner option and pre-configured web-server. See the examples/example.ml program for an example, which can be run as:

$ dune exec -- examples/example.exe --listen-prometheus=9090
If run with the option --listen-prometheus=9090, this program serves metrics at
http://localhost:9090/metrics
Tick!
Tick!
...

Unikernels can use Prometheus_app instead of Prometheus_unix to avoid the Unix dependency.