I was playing soulsilver the other day, and my togepi used metronome which ended up being spacial rend! So what is the possibility of that happening?
Well there are 467 moves in the soul silver and there are 24 moves that metronome cant use so since metronome randomly uses a move, it is 1/(467-24) = 1/443 which is about a .23% chance.
Legendary signature moves do not make a difference when considering metronome nor is any other really powerful move. A move like spacial rend has the same chance of appearing as a move like splash.