Switch to monotonic time for task logs
Switch to monotonic time for task logs

This matches the approach we take in utils [1]. Monotonic time is
better because it avoids weird negative results due to clock shift.

[1]: https://github.com/alphagov/notifications-utils/blob/5d18ebd79623e7d16bd48fbfe78ae632c0c0aaf1/notifications_utils/statsd_decorators.py#L14

alphagov/notifications-api,alphagov/notifications-api