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.

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summary: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.