For #12580 (http.Transport tracing/analytics)
Updates #13021

Change-Id: I126e494a7bd872e42c388ecb58499ecbf0f014cc
Reviewed-on: https://go-review.googlesource.com/22101
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>