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.

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.