[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

[sentry-warden]

set_attribute method does not exist on Span class, will raise AttributeError

The code assigns set_on_span = span.set_attribute when the span is an instance of Span, but the Span class (in sentry_sdk/tracing.py) does not have a set_attribute method. This will cause an AttributeError at runtime when _record_exception_on_span is called with a legacy Span object. The logic appears to be inverted - Span has set_data() while StreamedSpan has set_attribute().

Verification

Verified by reading sentry_sdk/tracing.py (Span class definition at line 225) - it has set_data() at line 590 but no set_attribute method. Confirmed via grep that 'set_attribute' has no matches in tracing.py. Also verified sentry_sdk/traces.py where StreamedSpan class at line 216 has set_attribute() at line 410.

Suggested fix: Swap the method assignments - use set_data for Span and set_attribute for StreamedSpan

Suggested change

	if isinstance(span, Span):
	set_on_span = span.set_attribute
	else:
	set_on_span = span.set_data
	set_on_span = span.set_data
	set_on_span = span.set_attribute

Also found at 1 additional location

• sentry_sdk/integrations/sqlalchemy.py:102-102

Identified by Warden code-review, find-bugs · LS6-SGE

[sentry-warden]

Fix attempt detected (commit 4b2147d)

The fix attempted to address the inverted logic but introduced the same error in reverse - it assigns set_attribute to Span (which doesn't have this method) and set_data to StreamedSpan (which has set_attribute), so the AttributeError on Span objects will still occur at runtime.

The original issue appears unresolved. Please review and try again.

_{Evaluated by Warden}