and the another question is, when should Callback event should be triggered? if the callback is triggered whenever a user see a feature, we’re going to include only users who see the feature to metrics calculation. Meanwhile, when the callback is triggered when users visit app, it will include all users who are using the app to metric calculation. which way is more practical way to do?

I think trigger when users see the feature make more sense, because we can see how many users see the feature we’re testing. But it will not cover all app users. And I think this can be fixed by running many experiments at the same time.