Google docs say:
This event is triggered only by versions of your app that include the Firebase SDK. Note: paid app-purchase revenue, subscription revenue (Android only), and refunds are not automatically tracked. Your reported revenue may differ from the values you see in the Google Play Developer Console. Events that are flagged as being invalid or as sandbox (test) are ignored. Only iOS events are flagged as sandbox. Learn more about testing Google Play billing.
In other words in Android only subscriptions events are not tracked automatically. Is there a way how to track them? Do you have any implementation examples?