[do not merge] feat: Span streaming & new span API

[sentrivana]

Introduce a new start_span() API with a simpler and more intuitive signature to eventually replace the original start_span() and start_transaction() APIs.

Additionally, introduce a new streaming mode (sentry_sdk.init(_experiments={"trace_lifecycle": "stream"})) that will send spans as they finish, rather than by transaction.

import sentry_sdk

sentry_sdk.init(
    _experiments={"trace_lifecycle": "stream"},
)

with sentry_sdk.traces.start_span(name="my_span"):
    ...

The new API MUST be used with the new streaming mode, and the old API MUST be used in the legacy non-streaming (static) mode.

Migration guide: getsentry/sentry-docs#16072

Notes

• The diff is huge mostly because I've optimized for easy removal of legacy code in the next major, deliberately duplicating a lot. I'll of course split it up to reviewable PRs once ready.
  • Chose to go with a new file and a new span class so that we can just remove the old Span and drop the new StreamedSpan in tracing.py as a replacement.
• The batcher for spans is a bit different from the logs and metrics batchers because it needs to batch by trace_id (we can't send spans from different traces in the same envelope).

Release Plan

• There will be prereleases for internal testing.
• We'll release the new API in a minor version as opt-in.
• In the next major, we'll drop the legacy API.

Project

https://linear.app/getsentry/project/span-first-sdk-python-727da28dd037/overview

[github-actions]

Semver Impact of This PR

⚪ None (no version bump detected)

📋 Changelog Preview

This is how your changes will appear in the changelog.
Entries from this PR are highlighted with a left border (blockquote style).

---

New Features ✨

Openai

• feat(openai): Set system instruction attribute for Responses API by alexander-alderman-webb in #5376
• feat(openai): Set system instruction attribute for Completions API by alexander-alderman-webb in #5358

Other

• feat(ai): Add original input length meta attribute by alexander-alderman-webb in #5375
• feat(openai-agents): Set system instruction attribute on gen_ai.chat spans by alexander-alderman-webb in #5370
• feat(transport): Report 413 responses for oversized envelopes by alexander-alderman-webb in #5380

Bug Fixes 🐛

• fix(ai): Keep single content input message by alexander-alderman-webb in #5345
• fix(arq): handle settings_cls passed as keyword argument by nc9 in #5393
• fix(dramatiq): cleanup isolated scope and transaction when message is skipped by frankie567 in #5346
• fix(google-genai): deactivate google genai when langchain is used by shellmayr in #5389

Internal Changes 🔧

• ci: migration to the new codecov action by MathurAditya724 in #5392
• ref: Replace set_data_normalized() with Span.set_data() for system instructions by alexander-alderman-webb in #5374

Other

• [do not merge] feat: Span streaming & new span API by sentrivana in #5317

• Update CHANGELOG.md by alexander-alderman-webb in 8517eb0a
• release: 2.51.0 by alexander-alderman-webb in 93e89e4c

---

_{🤖 This preview updates automatically when you update the PR.}

[github-actions]

Codecov Results 📊

✅ 13 passed | Total: 13 | Pass Rate: 100% | Execution Time: 6.83s

All tests are passing successfully.

❌ Patch coverage is 22.17%. Project has 14286 uncovered lines.

Files with missing lines (180)

File	Patch %	Lines
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---

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