How to call a function when google pay billing is renewed

Question

i am working on an streaming app project. i wish to use google pay subscription in order to get the package to access the video. i already implemented the google pay billing and acknowledge my subscription so when i tested it, the subscription is already working well. the problem is i can't detect when the subscription renewal happen so i can fire my function in order to add the package that the user should get. i already checked for google documentation but can't seem to find how to do this. i will really appreciate if someone can show me how to detect when the subscription renewal happening ?