database/sql: add additional Stats to DBStats

database/sql: add additional Stats to DBStats

Provide better statistics for the database pool. Add counters
for waiting on the pool and closes. Too much waiting or too many
connection closes could indicate a problem.

Fixes #24683
Fixes #22138

Change-Id: I9e1e32a0487edf41c566b8d9c07cb55e04078fec
Reviewed-on: https://go-review.googlesource.com/108536
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>