metrics: refactor counters

The metrics daemon had counter classes that represented
persistent, tagged values, and UMA stats.  To create some
derived UMA stats we need to separate persistency and UMA stats,
and the tagging doesn't help either.

This rewrite is supposed to keep the same functionality.

BUG=chromium:339588
TEST=ran and checked a few histograms
BRANCH=none

Change-Id: Ia1121ab2db391d71edffab9f52afe29ce17686ba
Reviewed-on: https://chromium-review.googlesource.com/188082


Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>