feat(asgi): Migrate away from event processor in span first

[sentrivana]

Description

In span first, there are no event processors. Therefore, we need to be able to set the data we were setting in event processors in some other way.

As we migrate our integrations one by one, this will be an exercise in whether it's possible to achieve this without some sort of callback/lifecycle hooks. So far, in ASGI, it seems we can get by by simply using scope.set_attribute() for setting request related data, and updating the segment name/source just before the span ends.

Adding this enables us to actually test the new functionality.

Issues

• Closes Event processor replacement for streamed spans #5722
• Closes https://linear.app/getsentry/issue/PY-2152/event-processor-replacement-for-streamed-spans

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[linear-code]

PY-2152 Event processor replacement for streamed spans

[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

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[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

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[github-actions]

Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 8.70s

📊 Comparison with Base Branch

Metric	Change
Total Tests	—
Passed Tests	—
Failed Tests	—
Skipped Tests	—

✨ No test changes detected

All tests are passing successfully.

❌ Patch coverage is 6.12%. Project has 14732 uncovered lines.
✅ Project coverage is 30.02%. Comparing base (base) to head (head).

Files with missing lines (2)

File	Patch %	Lines
asgi.py	15.69%	⚠️ 172 Missing
_asgi_common.py	14.49%	⚠️ 59 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    25.26%    30.02%    +4.76%
==========================================
  Files          189       189         —
  Lines        21007     21053       +46
  Branches      6862      6886       +24
==========================================
+ Hits          5306      6321     +1015
- Misses       15701     14732      -969
- Partials       432       477       +45

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