2.0

CHANGES:

• Add a new prometheus-eio serving package (@mtelvers @avsm #60)

  Prometheus_eio.callback is a cohttp-eio handler that serves the
  default registry at /metrics. It depends only on prometheus-reporter,
  so no Lwt or cohttp-lwt code is linked into an Eio application.

• Add a new prometheus-reporter package with the cohttp-free parts of
  prometheus-app (@ulrikstrid @avsm #39 #46)

  Prometheus_reporter renders the text format and registers the GC
  collectors. Prometheus_reporter_unix adds the process start-time metric
  and the Logs reporter. prometheus-app reexports both so existing code
  keeps working. Applications can now serve the rendered metrics with
  any web server of choice.

• Remove Lwt from the prometheus core (@mtelvers @avsm #60 #65)

  The deprecated Lwt functions are removed from the core package, and
  CollectorRegistry.collect now returns a snapshot directly.
  Lwt collectors are all in prometheus-lwt, whose interface is unchanged
  from the v1.4 release.

  Applications that collected via the core in an Lwt context should use
  Prometheus_lwt.CollectorRegistry.collect.

• prometheus-app: add Prometheus_unix.config to build a configuration
  without cmdliner (@avsm @talex5 requested by @Nymphium #44 #73)

• prometheus-app: allow serving a registry other than the default one.
  (@avsm #54)

  Prometheus_app.Cohttp(S).callback and Prometheus_unix.serve take an
  optional ?registry argument.

• prometheus-app: allow specifying the address as well as the port to bind
  to with --listen-prometheus.

  As well as a bare port number, the option now accepts tcp:HOST[:PORT]
  (with IPv6 addresses in square brackets, and the port defaulting to 9090)
  and unix:PATH for domain sockets, in the same syntax as capnp-rpc-unix.
  (@rbjorklin @avsm @talex5 #51 #72)