Metrics infrastructure for OCaml

Metrics provides a basic infrastructure to monitor and gather runtime metrics for OCaml program. Monitoring is performed on sources, indexed by tags, allowing users to enable or disable at runtime the gathering of data-points. As disabled metric sources have a low runtime cost (only a closure allocation), the library is designed to instrument production systems.

Metric reporting is decoupled from monitoring and is handled by a custom reporter. A few reporters are (will be) provided by default.

Metrics is heavily inspired by Logs.