This change provides a convenient way to monitor database connection pool.

Change-Id: I4b3757855b43f3b254acf9312e2a16e2f87840d0
Reviewed-on: https://go-review.googlesource.com/7950


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>