1.4

CHANGES:

Core/Lwt split:

• Add a new prometheus-lwt metrics package.
  (@avsm @mtelvers @talex5 #66 #67 #68 #69, reviewed by @dinosaure)

  This release is not a breaking change, but instead prepares for
  deprecation of Lwt in the core package so a future release can remove Lwt
  from the prometheus core. Users of the core's Lwt-typed functions
  should migrate to Prometheus_lwt now. Code using the new interface will
  keep working unchanged in future releases.

• Add synchronous variants of the timing helpers to the core. Metric recording
  is synchronous and these will become the only core timing helpers once the
  Lwt-typed ones move to prometheus-lwt.

• The replacement time-based functions no longer take a gettime argument.
  Instead, this is provided once by the new Prometheus.init function,
  which is called automatically when using Prometheus_unix.

To migrate existing code, add a dep on prometheus-lwt and rename following
the deprecation warnings from the compiler.

Bug fixes:

• Format floats with %.17g (@samoht @talex5 #62).
  The text exposition formatter printed metric values with %f,
  so any magnitude below ~5e-7 was reported as 0.000000.

Other changes:

• Remove Astring dependency (@talex5 #63).

• Remove Asetmap dependency (@talex5 #64).
  This changes the types of LabelSetMap and MetricFamilyMap slightly,
  which might affect custom reporters.