In a soup pot, combine and bring to a boil the chicken broth, lemon grass, lime leaves and ginger. Lower heat, cover and simmer for 10 minutes to infuse the broth with flavor. Strain the infused broth, discard the aromatics and return the broth to the pot. Add chicken, mushrooms, chiles, fish sauce, sugar, coconut milk, and lime juice and simmer for another 10 minutes until the chicken is cooked through. Finally season the soup with salt and pepper. Ladle into serving bowls and top each bowl with some cilantro leaves. Serve immediately.