Add per-host status code histogram metric

[arjan]

Added status code metrics histogram to be able to monitor HTTP requests per host.

[benoitc]

Closing as outdated. The file hackney_response.erl no longer exists - hackney has been refactored to use a gen_statem-based connection model (hackney_conn.erl). The metrics integration would need to be reimplemented for the new architecture. Thank you for the contribution!