fix(llmobs): clear meta_struct on user-processor span omit

When a user span_processor returns None to drop an LLMObs span, also
clear meta_struct["_llmobs"] off the underlying APM span. Previously
the early return in _prepare_llmobs_span_data left the LLMObs payload
in place. That's a no-op today (the LLMObs writer never enqueued, and
APM doesn't read meta_struct["_llmobs"] yet), but it becomes a leak
post-convergence (MLOB-4925): once APM ships meta_struct["_llmobs"] to
the LLMObs backend, an "omitted" span whose payload was left intact
would arrive at LLMObs anyway — exactly the data the user told us to
drop.

The scrub is unconditional: it overrides _DD_LLMOBS_TEST_KEEP_META_STRUCT
because the omit contract is "no LLMObs data on this span", regardless
of test mode. The env var only suppresses the kept-path scrub (where
data must survive for assertions).

Rewrites test_processor_omit_span to assert directly on the contract:
omit ⇒ _get_llmobs_data_metastruct(span) == {}, kept ⇒ payload intact.
The test no longer references llmobs_events, so the only remaining
llmobs_events callsite in tests/llmobs/ is
test_malformed_span_logs_error_instead_of_raising — which legitimately
checks "no event was emitted" for a malformed span and stays on the
existing fixture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Yun-Kim

ddtrace/llmobs/_llmobs.py

@@ -593,6 +593,10 @@ def _prepare_llmobs_span_data(self, span: Span, span_kind: Optional[str]) -> boo
         user_processed_span = self._apply_user_span_processor(span, llmobs_span)
         if user_processed_span is None:
             log.debug("LLMObs span %s dropped by user processor", span)
+            # Clear LLMObs payload so the underlying APM span carries no _llmobs meta_struct.
+            # Otherwise, when this span ships through the APM trace writer (post-convergence),
+            # the LLMObs backend would receive data the user's processor told us to drop.
+            span._remove_struct_tag(LLMOBS_STRUCT.KEY)
             return False
 
         _normalize_llmobs_meta(

tests/llmobs/test_llmobs.py

@@ -191,20 +191,19 @@ def _omit_span_processor(span: LLMObsSpan) -> Optional[LLMObsSpan]:
         return span
 
     @pytest.mark.parametrize("llmobs_enable_opts", [dict(span_processor=_omit_span_processor)])
-    def test_processor_omit_span(self, llmobs, llmobs_enable_opts, llmobs_events):
-        """Test that a processor that returns None omits the span from being sent."""
-        # Create a span that should be omitted
-        with llmobs.llm() as llm_span:
-            llmobs.annotate(llm_span, input_data="omit me", output_data="response", tags={"omit_span": "true"})
-
-        # Create a span that should be kept
-        with llmobs.llm() as llm_span:
-            llmobs.annotate(llm_span, input_data="keep me", output_data="response", tags={"omit_span": "false"})
-
-        # Only the second span should be in the events (suppression is a side effect on the wire,
-        # not on the meta_struct payload — keep this assertion on llmobs_events).
-        assert len(llmobs_events) == 1
-        assert llmobs_events[0]["meta"]["input"]["messages"][0]["content"] == "keep me"
+    def test_processor_omit_span(self, llmobs, llmobs_enable_opts):
+        """Test that a processor that returns None clears the LLMObs payload off the span."""
+        with llmobs.llm() as omit_span:
+            llmobs.annotate(omit_span, input_data="omit me", output_data="response", tags={"omit_span": "true"})
+
+        with llmobs.llm() as keep_span:
+            llmobs.annotate(keep_span, input_data="keep me", output_data="response", tags={"omit_span": "false"})
+
+        # Dropped: meta_struct["_llmobs"] is cleared so the APM-shipped span
+        # carries no LLMObs data to the backend.
+        assert _get_llmobs_data_metastruct(omit_span) == {}
+        # Kept: payload is intact for export.
+        assert get_llmobs_input_messages(keep_span) == [{"content": "keep me", "role": ""}]
 
     def test_ddtrace_run_register_processor(self, ddtrace_run_python_code_in_subprocess, llmobs_backend):
         """Users using ddtrace-run can register a processor to be called on each LLMObs span."""