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>