Set custom measurements on transactions

[marandaneto]

📜 Description

Users are allowed to set custom measurements

💡 Motivation and Context

Fixes #789
getsentry/team-mobile#31
And getsentry/team-mobile#51

💚 How did you test it?

📝 Checklist

• I reviewed submitted code
• I added tests to verify changes
• I updated the docs if needed
• All tests passing
• No breaking changes

🔮 Next steps

[codecov-commenter]

Codecov Report

Base: 89.99% // Head: 91.37% // Increases project coverage by +1.37% 🎉

Coverage data is based on head (a8a3c07) compared to base (c265311).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1011      +/-   ##
==========================================
+ Coverage   89.99%   91.37%   +1.37%     
==========================================
  Files         113        9     -104     
  Lines        3539      197    -3342     
==========================================
- Hits         3185      180    -3005     
+ Misses        354       17     -337     

Impacted Files	Coverage Δ
dart/lib/src/noop_sentry_span.dart
dart/lib/src/protocol/sentry_span.dart
dart/lib/src/protocol/sentry_transaction.dart
dart/lib/src/sentry_measurement.dart
dart/lib/src/sentry_span_interface.dart
dart/lib/src/sentry_tracer.dart
dart/lib/src/sentry.dart
...art/lib/src/http_client/failed_request_client.dart
dart/lib/src/protocol/sentry_level.dart
...art/lib/src/enricher/enricher_event_processor.dart
... and 94 more

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[philipphofmann]

Gave this a quick pass and added some comments on the API.

[brustolin]

LGTM