+ Plug it in ValueMetric as the frist step as we often need to normalize the values
by the duration of condition being true.
E.g., Coulomb counter drop / screen_on_and_on_battery_time
Having a simple timer inside the metric will eliminate the need of creating
another duration metric and joining them on server side.
Test: statsd_test
Fix: 130048020
Change-Id: Ie377dc6de917176ab07843e0dfd8506ed1d5db75
