fix(tracing): nest ddtrace wrapper under the active context (child_of)

ddtrace start_span does not auto-parent (unlike OTel): start_span(name) mints a
new ROOT trace every call, so a turn's business spans scattered across N Datadog
traces (verified live: 52 spans -> 52 distinct obs_trace_ids). Pass
child_of=current_trace_context() so wrappers nest under the request/turn trace
and roll up into one trace; obs_span_id stays distinct per step.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: NiteshDhanpal

src/agentex/lib/core/tracing/obs_span.py

@@ -116,9 +116,15 @@ def _open_ddtrace_span(
     try:
         # Only wrap when ddtrace is actually tracing the request; otherwise a
         # wrapper would be an orphan root trace in an un-instrumented process.
-        if tracer.current_trace_context() is None:
+        ctx = tracer.current_trace_context()
+        if ctx is None:
             return None
-        span = tracer.start_span(name, activate=True)
+        # child_of=ctx is load-bearing: ddtrace's start_span does NOT auto-parent
+        # to the active span (unlike OTel), so start_span(name) alone mints a NEW
+        # root trace every call -- scattering a turn's business spans across N
+        # Datadog traces. Parenting to the active request/turn context rolls them
+        # into one trace while obs_span_id stays distinct per step.
+        span = tracer.start_span(name, child_of=ctx, activate=True)
         if business_span_id:
             span.set_tag(_ATTR_BUSINESS_SPAN_ID, business_span_id)
         if business_trace_id:

tests/lib/core/tracing/test_obs_span.py

@@ -95,15 +95,17 @@ def finish(self):
 
 def _install_fake_ddtrace(monkeypatch, *, active=True, trace_id=0xABC, span_id=0xFF):
     record: dict = {"span": None, "started": []}
+    ctx_obj = object() if active else None
+    record["ctx"] = ctx_obj
 
-    def start_span(name, activate=False):
+    def start_span(name, child_of=None, activate=False):
         span = _FakeDDSpan(name, trace_id, span_id)
         record["span"] = span
-        record["started"].append((name, activate))
+        record["started"].append({"name": name, "child_of": child_of, "activate": activate})
         return span
 
     tracer = types.SimpleNamespace(
-        current_trace_context=lambda: (object() if active else None),
+        current_trace_context=lambda: ctx_obj,
         start_span=start_span,
     )
     fake_ddtrace = types.ModuleType("ddtrace")
@@ -204,7 +206,12 @@ def test_dd_only_with_active_ctx_opens_named_span(self, monkeypatch):
 
         assert handle is not None
         assert record["span"].name == "rocket.tool.fetch"
-        assert record["started"] == [("rocket.tool.fetch", True)]  # activated
+        started = record["started"][0]
+        assert started["name"] == "rocket.tool.fetch"
+        assert started["activate"] is True
+        # child_of is the active request/turn context -> the wrapper nests under
+        # it instead of minting a new root trace (ddtrace does not auto-parent).
+        assert started["child_of"] is record["ctx"]
         assert handle.correlation == {
             "obs_trace_id": "00000000000000000000000000000abc",
             "obs_span_id": "000000000000000000ff"[-16:],