This change exposes a function to extract rate limit duration from a client error using Retry-After response header. Author: David Calavera <david.calavera@gmail.com>. Fixes golang/go#19304. Change-Id: Iec9cfab398b84c6f216b95d3265ffad1ce2f29a7 Reviewed-on: https://go-review.googlesource.com/37463 Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>