driver: make metrics self-describing

The current metrics lack units and often don't really describe what
they're measuring. Change all of the metrics to follow the "value
units" guidelines in the standard benchmark format [1].

[1] https://github.com/golang/proposal/blob/master/design/14313-benchmark-format.md

Change-Id: I895f204d0f25702e0172070a533d0356d1149bea
Reviewed-on: https://go-review.googlesource.com/33582
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>