Baggage.serialize(include_third_party=True) leaves a trailing comma when there are no third-party items #8120

How do you use Sentry?

Sentry Saas (sentry.io)

Version

2.73.0 (current master)

Steps to Reproduce

Baggage.serialize(include_third_party=True) in sentry_sdk/tracing_utils.py appends self.third_party_items unconditionally. When there are no third-party items it is "", so ",".join(items) ends with an empty member.

from sentry_sdk.tracing_utils import Baggage

baggage = Baggage({"trace_id": "771a43a4192642f0b136d5159a501700", "sample_rate": "0.5"})
print(baggage.serialize(include_third_party=True))

This is reachable from the Celery integration: _update_celery_task_headers calls combined.serialize(include_third_party=True) when it merges incoming and Sentry baggage, so the common Sentry -> Celery -> Sentry case (incoming baggage with only sentry- items) sends the outgoing baggage header with the stray comma.

Expected Result

No trailing comma when there are no third-party items:

sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rate=0.5

The output with third-party items present stays as it is today.

Actual Result

sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rate=0.5,

Under the W3C Baggage spec the trailing comma is an empty list-member. Sentry's own parser tolerates it, but a strict W3C / OpenTelemetry baggage parser on a downstream service can reject or mishandle the header.

Proposed fix: only append third_party_items when it is non-empty (if include_third_party and self.third_party_items:). I have this ready with a test in #8119 and am happy to adjust the approach.