content: add introducing http tracing post

Updates golang/go#17152.

Change-Id: I08d4174ba62f6dc4028bd17a911b92fc373b81e4
Reviewed-on: https://go-review.googlesource.com/29685
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
3 files changed