emit metrics about range completeness as events

A nice source of truthful delay information.
Not sure if that's much better. It adds connotations as if the logic were something specific to Range*, which it isn't. Just timedelta.total_seconds is new in Python 2.7. It would maybe be good to have some kind of "utility"/"compat"/"future" module/convention to hold backported functionality like this; not sure what's the best practice.