Package trace implements tracing of requests.
It exports an HTTP interface on /debug/requests,
and higher level packages (e.g. an RPC system)
can construct and annotate traces.

Change-Id: I67e981ed28bc6a15e4ad5a02217500505896ce1c
Reviewed-on: https://go-review.googlesource.com/10741Reviewed-by: Nigel Tao <nigeltao@golang.org>