Firebase Analytics on iOS Extensions is not working correctly

### DAU are wrong on iMessage extension

On iOS extensions (iMessage and Keyboard) the numbers of Firebase DAU looks wrong.
I've compared them with Fabric DAU and the discrepancy is 10x lower in Firebase vs Fabric. I know they are different product etc, but the strange part is that on the main app the numbers are almost exactly the same.
Moreover, I've been checking some of the main app events that should trigger a user_engagement event.
**session_start** (automatically fired by Firebase), compared to the number of **users** that triggered that event result to be 15x bigger than the DAU.
While on the main app DAU and users triggering the **session_start** is the same number.

### Environment:

* Xcode version: iOS11+
* Firebase SDK version: 5.20.2
* Firebase Component: Analytics
* Component version: 5.8.1

#### Steps to reproduce:

Observing the live stream you can noticed the the user_engagement is almost never fired on iMessage extension.