The tokens created by Krenko's triggered ability aren't attacking. Because all attackers are chosen at once, a token created this way can't attack, even if it gains haste.

If Krenko leaves the battlefield after its ability has triggered but before it resolves, you don't put a +1/+1 counter on it, but you do use its power as it last existed before it left the battlefield to determine how many Goblin tokens to create.