mod_prometheus: Invoke stats collection if in 'manual' mode Since 10d13e0554f9 a special value for statistics_interval "manual" exists, where a module is expected to invoke processing in connection to collection of stats. This makes internal collection and exporting to Prometheus happens at the same time with no chance of timers getting out of sync.

This module helps reduces power usage of inactive mobile clients while
another client is actively used. E.g. in the case of chatting on a
desktop computer, instantly pushing messages to a phone in someones
pocket is not the best use of radio time.

Works with [mod_csi_simple][doc:modules:mod_csi_simple] which is
included with Prosody since 0.11