[FR]: Performance: Allow adding custom attributes to built-in traces

## What feature would you like to see?

Firebase performance is tracking the number of built-in events, such as

- App lifecycle (_app_start, _app_in_background, _app_in_foreground)
- Network requests
- Time spent on a screen

Currently, it is not possible to assign custom attributes to those traces. It would be great to allow this.

### **Firebase Product(s)**

Performance