How to choose and set up warning threshold for TM contention lock in database

Hello,

We have TM contention in our database instance sometimes and users always need to have  their sessions killed because their session hung. So we need to set up a warning in EM12c metric so that DBAs can know this issue before hand. What metric should I choose and what value I need to set up for raising a warning incident for TM contention issues in database? We use EM 12.1.0.2. Our database is 10.2.0.4 EE.

Thank you for your inputs!