Use perf_counter for timing metrics
As of 3.3 the Python time library provides a perf_counter
[1] method to measure
durations between two points in time. Rather than use absolute timestamps as we
were before, switch to using perf_counter
to measure the times between the
start and stop of the timer metric.
Add additional tests to ensure conversions work as expected.
[1] https://docs.python.org/3/library/time.html#time.perf_counter