fix uses of Instant::sub

[hawkw]

It looks like clippy's disallowed methods can't detect the use of
disallowed operators, so #1456 misses a bunch of uses of the -
operator on Instants, mostly in metrics, tap, and the log timestamp
formatter.

This fixes all of them that I could find.

[hawkw]

i missed that a couple of those were actually <Instant> - <Duration> instead, which broke when changed to saturating_duration_since (and, the non-saturating subs were legit in that case since the Duration is not going to time travel)

[olix0r]

Excellent, thanks!