rate: use time.Since instead of time.Now().Sub
rate: use time.Since instead of time.Now().Sub

Change-Id: I140abf9d9283edb7259c9aa13ee69f3ca33181e6
Reviewed-on: https://go-review.googlesource.com/c/time/+/325609
Trust: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/time