Upekkha. Do not complicate things brother. Metta are attained in understanding nature of this world. Whatever Buddha speaks, he speaks about one thing (Nibbana) from different angles. You cultivate metta by getting rid of tanha and avijja. Once you get rid of cravings, ignorance and you see there is no point doing bad things – natural niramisa sukha arises which gives birth to metta, karuna etc.
Any type of “keeping something” is still anariya bhavana and anariya thinking.