Output duration of the test

[pratikmallya]

It would be nice if the summary included the total wall time that a test takes. e.g. a duration field:

{"level":"info","completed":19886,"failed":114,"total":20000,"rps":26,"statusCodes":{"200":19884,"500":2},"time":"2020-07-13T14:47:08Z","message":"stats", "duration":"100us" }

[forestgagnon]

Since each log line is purely a stat log at a fixed interval, what if each line listed the total time ravager has been running up to that moment? And a final log at the end with the total run time.