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

[sentrivana]

Introduce a new tracing.start_span() with a streamlined signature to eventually replace the original sentry_sdk.start_span() and sentry_sdk.start_transaction() APIs:

import sentry_sdk

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

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

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

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: Coming soon in #5318

TODOs

• Profiling
• ...

Notes

• 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).
• 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.

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, and optionally point the top-level sentry_sdk.start_span() to the new 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)

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Internal Changes 🔧

• ci: Fix path in AI integration tests by alexander-alderman-webb in #5347

Other

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

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