All the moves have the same chance.
Assist, chatter, copycat, counter, covet, destiny bond, detect, endure, feint, focus punch, follow me, helping hand, me first, mimic, mirror coat, mirror move, protect, sketch, sleep talk, snatch, struggle, switcheroo, thief,and trick are the only moves that Metronome can't choose.
It's a very unreliable move; competitively speaking, nobody will use it, so it's not something you have to worry about. Even if you had to, there would be no way to prepare for it-it's too unpredictable.