In a medium saucepan, heat chicken stock, tom yam paste, lime leaves, lemon grass, lime juice, fish sauce, chiles, and sugar. Once it has come to a boil, add mushrooms and simmer for 2 minutes. Add shrimp and scallions and cook for 2 to 3 minutes allowing the shrimp to cook but still be tender. Sprinkle with cilantro and serve immediately. Place extra cilantro on the table for people to add more.