Proof of Concept: Frame Tracking

[ueman]

This PR shows how one could add the 'total frames' measurement for transactions.
The current code needs some more polishing before it's ready for release.
I'm currently not able to finish this, so I would appreciate it, if someone else would take it over the finishing line.

This also seems to require some (bigger?) changes in the transaction/span API. It's also tricky to get the total frame number with auto closing navigation transaction, because there's no callback.

[codecov-commenter]

Codecov Report

Merging #699 (f49a663) into main (6206e85) will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #699      +/-   ##
==========================================
+ Coverage   90.70%   90.96%   +0.26%     
==========================================
  Files         104        8      -96     
  Lines        3336      155    -3181     
==========================================
- Hits         3026      141    -2885     
+ Misses        310       14     -296     

Impacted Files	Coverage Δ
dart/lib/src/protocol/sentry_transaction.dart
dart/lib/src/sentry_tracer.dart
.../lib/src/navigation/sentry_navigator_observer.dart
flutter/lib/src/sentry_asset_bundle.dart
dart/lib/src/sentry_user_feedback.dart
flutter/lib/src/sentry_flutter.dart
dart/lib/src/protocol/sentry_browser.dart
flutter/lib/src/sentry_flutter_options.dart
dart/lib/src/http_client/sentry_http_client.dart
...ib/src/environment/_web_environment_variables.dart
... and 86 more

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[marandaneto]

@denrase you probably want to check that out since we'd need something similar for the Mobile vitals.

[ueman]

Maybe a ticker is a better candidate than this? See https://api.flutter.dev/flutter/scheduler/Ticker-class.html

[ueman]

This feature was implemented with a different approach.