Fixes test_metrics bug in FixtureReporter

 * Prevents stop_test_metrics from running if
   start_test_metrics wasn't previously called.
 * Raises a warning and logs appropriate messages
   for the error case.
 * This issue is caused by tests written with drivers
   that have a set-up/tear-down mechanic in their test
   fixtures.  In the case where a set-up method calls
   start_test_metrics and a tear-down method calls
   stop_test_metrics, any inheriting class that
   overrides the set-up but not the tear-down will
   cause this issue.  (Also, any methods that override
   but don't call super() on the set-up method)

Change-Id: I3e70009c02cc1a2465ad20600f0a73dd8c5e6ac0
Merge "Fixes test_metrics bug in FixtureReporter"