If, during your declare attackers step, a creature that must attack if able is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under your control continuously since the turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having a creature attack, you're not forced to pay that cost, so it doesn't have to attack in that case either.

The number of attacking Goblins is counted as the last ability resolves, and the bonus is locked in at that time.

Although Goblin Rabblemaster doesn't force itself to attack, if you control two of them, they'll force each other to attack if able.