If you use Sketch on a Pokémon that just used Metronome, the sketched move is going to be the move that Metronome calls, but how can I sketch Metronome?
Depends on what you're playing.
If you're in a gen 2 game, it cannot be done.
In gen 3, you do it by sleeping the opponent and if they use metronome, it will fail, and you can sketch it.
In gen 4+, do literally anything that can make your opponent fail. (sleep, paralyze, taunt.) If metronome fails, you sketch it.