After you unlock Metal Sonic by collecting all 130 emblems in the game you can select him in Trial mode. You simply have to beat Level A on all of his stages. These are identical to the stages and objectives you had when getting Level A with Sonic. Metal Sonic is just a model swap for sonic, he functions exactly the same (if there are any differences besides his run animation, I didn't notice them).
If you were able to unlock him, you should have no problem beating the sonic levels 1 more time.