Initialize this scoped guard to record elapsed time using the specified collector. Optionally specify the timeUnits in which to report elapsed time. If collector is 0 orcollector->category().enabled() == false, this object will be inactive (i.e., will not record any values). The behavior is undefined unless collector == 0 || collector->metricId().isValid(). Note that timeUnits indicates the scale of the double value reported by this guard, but does not affect the precision of the elapsed time measurement.