fix(tracing): keep W3C tracestate dd= member within 256 chars

[julinvictus]

Description

w3c_get_dd_list_member keeps the dd= list-member of the W3C tracestate header under 256 characters, but it only counted the tag text. Three things that end up in the header were not counted:

• the ; separators between tags (1 char each)
• the p:<16 hex>; parent id field, prepended at injection time by w3c_tracestate_add_p / the native build_tracestate (19 chars)
• the dd= prefix (3 chars)

So the injected member could exceed the limit by roughly 30 characters. This now shows up in practice because LLM Observability propagates more _dd.p.llmobs_* tags (llmobs_sid, llmobs_pagent_span_id, llmobs_pagent_name). With a typical set of those tags, the injected dd= member was 285 characters. Proxies that enforce the W3C limit drop the whole tracestate header, which breaks LLM Observability trace linking across services.

The fix:

• Reserves space for dd= and p:<16 hex>; up front (_W3C_DD_LIST_MEMBER_RESERVED_LEN).
• Counts the required fields (s:, o:, t.dm:, t.usr.id:) as they are written, separators included.
• Counts 1 extra char for the ; before each optional _dd.p.* tag.

As before, tags that don't fit are dropped whole and never truncated.

Before (285 chars):

dd=p:10a3f888ec177dd0;s:1;o:rum;t.llmobs_parent_id:10707708093473052057;t.llmobs_trace_id:141883241939442010658227520463581072062;t.llmobs_ml_app:datasciencetiger;t.llmobs_sid:11a0eec0-ad0a-4f4d-8ae8-7f358ed1b153;t.llmobs_sr:1;t.llmobs_sd:1;t.llmobs_pagent_span_id:14260131734498464268

After (240 chars, t.llmobs_pagent_span_id dropped because it no longer fits):

dd=p:61d200da4b9294a3;s:1;o:rum;t.llmobs_parent_id:10707708093473052057;t.llmobs_trace_id:141883241939442010658227520463581072062;t.llmobs_ml_app:datasciencetiger;t.llmobs_sid:11a0eec0-ad0a-4f4d-8ae8-7f358ed1b153;t.llmobs_sr:1;t.llmobs_sd:1

Testing

• Reproduced the overflow on v4.15.2 by injecting headers from an active span with the LLM Observability tags shown above (285 chars). With this patch applied to the same version, the member is 240 chars. The diff applies cleanly to v4.15.2.
• Ran the existing w3c_get_dd_list_member / tracecontext / inject tests from tests/tracer/test_utils.py and tests/tracer/test_propagation.py with and without the patch. The patch introduced no new failures.

Risks

• p: space is reserved even when no p: field is added (no active Datadog span and no known last parent id). In that case up to 19 fewer characters are used for propagated tags than strictly allowed.
• When the budget is exceeded, the tags that come last in the trace's meta order are dropped. That ordering behavior is unchanged, but more tags can now be dropped than before, which is required to respect the limit. Today, with the LLM Observability tags, llmobs_pagent_span_id is the one most likely to be dropped.

Additional Notes

Opened from a fork, so some required CI can't run here and will need a maintainer to mirror the branch.

🤖 Generated with Claude Code

[Yun-Kim]

Small nits but LGTM otherwise!

[Yun-Kim]

Might be more clear and not require the comment if we change to this:

Suggested change

	_W3C_DD_LIST_MEMBER_RESERVED_LEN = 3 + len(W3C_TRACESTATE_PARENT_ID_KEY) + 1 + 16 + 1
	_W3C_DD_LIST_MEMBER_RESERVED_LEN = len("dd=") + len(f"{W3C_TRACESTATE_PARENT_ID_KEY}:{0:016x};")

[Yun-Kim]

Comments are unnecessary here

Suggested change

	# The 256 char limit applies to the whole "dd=" list-member as it goes on the wire, so count the
	# "dd=" prefix, the ";" separators, and the "p:<16 hex>;" field prepended at injection time.

[Yun-Kim]

Suggested change

	next_tag_len = len(next_tag) + (1 if tags else 0)
	# account for ; before next tag entry
	next_tag_len = len(next_tag) + (1 if tags else 0)

[Yun-Kim]

Suggested change

	header, which broke LLM Observability trace linking across services. Propagated tags that do not fit
	are now dropped whole.
	header. Propagated tags that do not fit are now dropped whole.