Confused as to why stockfish says +2.2

The short version of the story seems to be that black has overextended his center and it will now collapse hard after 10. c4!. Which looks like it will end up winning at least one of blacks center pawns with activity to follow. You can for example see how the bishop on g2 comes to life with potential threats against b7.

Whites main threat is to follow up with 11. Bxf6 Qxf6 12. Nbc3 (or Nec3) when black cannot avoid losing his d5 or e4 pawn with tempo.

Another important point is that after 10... dxc4 11. Bxf6 Qxf6 12. Nbc3! white threatens to trap the rook on a8 with Nxe4 followed by Nxd6 and Bxb7.

The end of that game... It's impolite to play with your food. Put the guy out of his misery already :D

Its also polite to resign in a lost position.

It's only impolite to even think about taking others the right to decide on with point they want to resign. Blunders and even stalemate happens way more often, than you would think. Fighting spirit is never impolite, playing with your opponents hope on the other side is. But the good thing is people who do this, often run into time problems or stalemate and thats just fair.

Thanks PastaProgrammer! I'll try to understand more of how the center collapses.

I'm not allowed to resign until I turn 30 according to my chess coach

It's your game of chess. Resign when you're good and ready. Ignore advice about being "polite". Your coach has it right.