Have scrape time as a pseudovariable, not a prometheus variable.

This ensures it has the right timestamp, and is easier to work with.

Switch sd variable away from 'outcome', using total/failed instead.
We've been using either `duration` or `latency` for durations. I recently agreed with @xla that we should standardize on `duration` for durations. Also, `time` might also be confused with a time when something happened (timestamp), not a duration. So how about: `scrape_duration_seconds`.

Hmmm, also: no namespacing at all on this metric? I know it's intentional (like in `up`), but I wonder if it should still have `prometheus` in it? I guess because it'll actually be attached to the scraped job, maybe that would be wrong. Yeah, I guess so...