Support common alerting fields in FieldValueProvider

[jan-auer]

Implement support for more fields in the FieldValueProvider so that alert and dashboard metrics can specify them in conditions. This issue focuses on the most commonly used fields:

Fields

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1. measurements
2. breakdowns (in-product: spans.*)
3. transaction duration
4. core attributes: dist, extra.*,
5. user.{email, username, ip}
6. release.version
7. generic tags

PRs

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1. feat(protocol): Add access to more fields via FieldValueProvider #2259
   +0
   
2. feat(metrics): Support access to structured release fields #2276
   +0
   
3. feat(sampling): Add more fields to FieldValueProvider #2293
   +0