Gambas al Ajillo (Spanish Garlic Shrimp) Recipe

1 Lb shell-on shrimp (21-25 per pound)
4 large garlic cloves
3/4 tsp diamond crystal kosher salt (or to taste)
1/4 tsp sugar
1/2 cup olive oil
pinch of red chili flakes
squeeze of lemon
chopped parsley for garnish