metrics library: convert to proper C++/libbase

This mostly converts fprintfs to proper logs, char* to
std::string wherever appropriate.

BUG=chromium:355796
TEST=unit tests

Change-Id: Ieb1cb110be5e281b7e0c764a0dfce895f33d4a3c
Reviewed-on: https://chromium-review.googlesource.com/199610


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