message: Document histogram bucket type change
diff: @@ -15,6 +15,11 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  common practice of looking into the current directory first before looking for
  a system-wide configuration file.
  +### Changed
  +
  +- Counters in histogram buckets are now always integers, independently on the
  + value type of the histogram. Buckets can never increase by fractional values.
  +
  ### Deprecated
  - All `parse` function overloads in `actor_system_config` that took a custom
  