Adding GA is essentially not possible. As it would need to set a Cookie and you can’t/shouldn’t set cookies. (Cookies are blocked iirc but I think localstorage is allowed not sure/can’t confirm off hand)
Further more it would be a nightmare from the security and privacy policy perspective. Due to the fact Twitch would technically need to make changes to it’s privacy policy in order to allow third party GA in an extension and fun with opt’ing etc. (Especially in a GDPR world)
What does exist is the Extensions Analytics you can find here: