I am trying to wrap my head around the Firebase User Engagement Calculation. I am doing a lot of debugview testing now, and as the online documentation states (https://support.google.com/firebase/answer/6317485), Firebase is firing the user_engagement event by default on a periodic basis within a session while the app is in the foreground.
What I have seen thusfar is that user_engagement is fired quite reliably in the beginning and end of an engagement (this can be occurring multiple times in a specific session).
Scenario: Say for instance I started my session and exited the app, but returned again within the the same 30 minutes time slot. The session_start won't fire again, but only user_engagement. This is quite clever, because I can actually then query first and last engagement experienced within this scenario.
My question is: How would I actually then query the difference between the beginning of a user_engagement and end of the user engagement ? Currently engagement_time_msec allows me to measure: The additional engagement time (ms) since the last user_engagement event (https://support.google.com/firebase/answer/7061705?hl=en), and not since the beginning of the user_engagement. What if I wanted to measure engagement from the first engagement and last engagement, assuming my specific scenario describe earlier. The idea of this question is to ask how would an example query look like to address this question.