docs: Document span filtering migration to span first

[harryautomazione]

What

• Adds documentation to MIGRATION_GUIDE.md explaining how to filter individual spans in the experimental "Span First" (streaming) mode.
• Provides code examples for using the experimental ignore_spans configuration with String, Regex, and Dict rules.

Why

• In transaction-based mode, spans are filtered in before_send_transaction.
• In Span First mode, spans are streamed individually and bypass before_send_transaction.
• Fixes Document span filtering migration #5363.

How

• Added a section ## Migrating Span Filtering to Span First in MIGRATION_GUIDE.md.
• Described the difference between modes and how _experiments={"ignore_spans": [...]} can be used.
• Verified that rules (String, Regex, and Attributes) match internally using is_ignored_span().

Testing

• Created a test script to verify is_ignored_span() behavior with multiple rule types (exact match, dictionary, attribute match).
• Confirmed child spans inherit the parent's ignored state structure.

Risks / Impact

• None, as it is documentation-only for an existing experimental config.

Checklist

• Linked the relevant issue (Fixes Document span filtering migration #5363).
• Described problem, solution and impact.
• Added or updated tests where appropriate (Verified logic with scratchpad tests).
• Verified no secrets or sensitive data are introduced.

[github-actions]

Semver Impact of This PR

🟢 Patch (bug fixes)

📋 Changelog Preview

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Entries from this PR are highlighted with a left border (blockquote style).

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New Features ✨

• (logging) Separate ignore lists for events/breadcrumbs and sentry logs by sl0thentr0py in #5698

Bug Fixes 🐛

Anthropic

• Set exception info on streaming span when applicable by alexander-alderman-webb in #5683
• Patch AsyncStream.close() and AsyncMessageStream.close() to finish spans by alexander-alderman-webb in #5675
• Patch Stream.close() and MessageStream.close() to finish spans by alexander-alderman-webb in #5674

Documentation 📚

• Document span filtering migration to span first by harryautomazione in #5710

• Add note on AI PRs to CONTRIBUTING.md by sentrivana in #5696

Internal Changes 🔧

• Add -latest alias for each integration test suite by sentrivana in #5706
• Use date-based branch names for toxgen PRs by sentrivana in #5704
• 🤖 Update test matrix with new releases (03/19) by github-actions in #5703
• Add client report tests for span streaming by sentrivana in #5677

Other

• Update CHANGELOG.md by sentrivana in #5685

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[harryautomazione]

This Pull Request was a duplicate of work bundled elsewhere by mistake. Thank you to @sentrivana for closing it and keeping the board clean. Sorry for the noise!