[tracing] Make run_metric script requires at least a single metric to run
[tracing] Make run_metric script requires at least a single metric to run

Currently if one runs "tracing/bin/run_metric <trace>", it doesn't report error.

Review-Url: https://codereview.chromium.org/2154213005

catapult-project/catapult,sahiljain/catapult,benschmaus/catapult,sahiljain/catapult,catapult-project/catapult-csm,benschmaus/catapult,benschmaus/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,sahiljain/catapult,catapult-project/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult-csm,sahiljain/catapult,benschmaus/catapult,catapult-project/catapult-csm,catapult-project/catapult,sahiljain/catapult,sahiljain/catapult