>When this item is held by a Pokémon, if the holder uses a move multiple times consecutively, its power is boosted by 10% of its original power up to Black & White, and 20% of its original power from Black & White. The effect stops increasing once it hits 100% and resets if another move is used.
According to Google, consecutive means "following continuously," which would mean that every moves after the first move would be boosted. Also, it would be impossible for the first move to be boosted because the game can't tell if you are going to use the same move after you use the first move.
So no, the first move used would not get the 20% boost from the Metronome.