Correction: You don’t need to keep the metronome full, it just needs to have the white bar take up some space. I made an alt and played half metronome the entire time and got it.
I somehow managaed to do it on the new map, Copenhagen. I sat behind the team and played the fife. I don't think the badge gives anything (if it does, correct me pls) (there was like 2-3 bombers in every room and I somehow was never hit once, and there was a drummer with us who was being attacked left and right)