metrics_daemon: log to syslog

Also log to stderr when not running as daemon.

BUG=chromium:364700
TEST=manually verified in both daemon and non-daemon modes

Change-Id: If62f70c1d327a5ecaf3075b6689b83a1494c09d5
Reviewed-on: https://chromium-review.googlesource.com/199380


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