Initialize this scoped guard to record elapsed time using the specified metric. Optionally specify the timeUnits in which to report elapsed time. If metric‐>isActive() is false, this object will also be inactive (i.e., will not record any values). The behavior is undefined unless metric is a valid address of a Metric object. 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.