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

❌ Patch coverage is 44.82759% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.12%. Comparing base (c265311) to head (a8a3c07).
⚠️ Report is 1451 commits behind head on main.

Files with missing lines	Patch %	Lines
dart/lib/src/sentry_measurement_unit.dart	40.00%	6 Missing ⚠️
dart/lib/src/sentry_tracer.dart	0.00%	4 Missing ⚠️
dart/lib/src/protocol/sentry_transaction.dart	25.00%	3 Missing ⚠️
dart/lib/src/protocol/sentry_span.dart	0.00%	2 Missing ⚠️
dart/lib/src/noop_sentry_span.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1011      +/-   ##
==========================================
+ Coverage   89.99%   90.12%   +0.12%     
==========================================
  Files         113      114       +1     
  Lines        3539     3554      +15     
==========================================
+ Hits         3185     3203      +18     
+ Misses        354      351       -3     

☔ View full report in Codecov by Sentry.
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[philipphofmann]

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

[brustolin]

LGTM