Name: prometheus_usage_update_freq
Type: DURATION
Default-Value: NULL
Last-Modified: 3.12.0

Frequency in at which promstatsd should re-collate its usage statistics
report.  Note that this report is relatively expensive to produce.  If not
set, usage statistics are not reported.

Best to make this a multiple of *prometheus_service_update_freq*.

If no unit is specified, seconds is assumed.
