Add missing measurements units

[marandaneto]

📜 Description

Not as nice as SentryMeasurementUnit.duration.milliSecond as some other languages, but it's a limitation of the enum type in Dart in the current min. supported version, we can refactor that in the future.

Changing it to a plain String would be a breaking change and would have a poor UX because we'd need to document all the possible values.

The current limitation is that you can only set the unit with one of the enum values, which is well covered for most of the cases.
In case somebody wants to use their own metric unit, it's currently not supported, we can refactor this and bump the Dart version if needed.

💡 Motivation and Context

Reason: getsentry/sentry-java#2260 (comment)

💚 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

[marandaneto]

Limitation before Dart 2.17 and we cannot do breaking changes just yet.
It's either this or an open String, but this has better UX, so users don't need to find out the units by themselves via docs.

[codecov-commenter]

Codecov Report

Base: 90.26% // Head: 90.48% // Increases project coverage by +0.21% 🎉

Coverage data is based on head (fad9d52) compared to base (812d379).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1033      +/-   ##
==========================================
+ Coverage   90.26%   90.48%   +0.21%     
==========================================
  Files         114      114              
  Lines        3556     3572      +16     
==========================================
+ Hits         3210     3232      +22     
+ Misses        346      340       -6     

Impacted Files	Coverage Δ
dart/lib/src/sentry_measurement_unit.dart	100.00% <100.00%> (+60.00%)	⬆️

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[philipphofmann]

Maybe there is a different way of fixing it.

[philipphofmann]

LGTM, as discussed on a call with @marandaneto, that Dart doesn't offer inheritance for enums or method overloads, and that's why we put all types into one enum.