This is an updated version of golang.org/cl/96395, with the fix to
TestUserSpan.

This reverts commit 7b6f6267e90a8e4eab37a3f2164ba882e6222adb.

Change-Id: I31eec8ba0997f9178dffef8dac608e731ab70872
Reviewed-on: https://go-review.googlesource.com/98236
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>