refactor(tracing): probe expected backend first in warn_on_backend_drift; rewrap docstring

- warn_on_backend_drift probes the expected backend first and returns early when
  it is live, so the healthy common path skips the second probe -- and avoids
  re-attempting the import of a backend that isn't installed (failed imports
  aren't cached in sys.modules, so the finder cost otherwise recurs every span).
  Behavior is unchanged.
- Rewrap the tag_ambient_obs_span docstring paragraph a prior edit left with one
  overlong line.

Addresses review nits.

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

Author: NiteshDhanpal

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

@@ -119,29 +119,29 @@ def warn_on_backend_drift(expect_otel: bool = False) -> None:
 
     Not a hard failure: obs stays fail-open (the caller still reads and falls back,
     so no correlation is lost). The warning is deduped per direction, so a standing
-    mismatch logs once, not once per span. Never raises."""
+    mismatch logs once, not once per span. Probes the expected backend first and
+    returns early when it is live, so the healthy common path never touches the
+    other backend. Never raises."""
     try:
-        otel = _lgtm_ids()
-        ddt = _ddtrace_ids()
         if expect_otel or get_obs_mode() == LGTM:
-            expected, expected_live = "otel", otel
-            other_live = ddt
+            expected, expected_probe, other_probe, actual = "otel", _lgtm_ids, _ddtrace_ids, "ddtrace"
         else:
-            expected, expected_live = "ddtrace", ddt
-            other_live = otel
-        if expected_live is None and other_live is not None:
-            actual = "ddtrace" if expected == "otel" else "otel"
-            if (expected, actual) not in _WARNED_DRIFT:
-                _WARNED_DRIFT.add((expected, actual))
-                _log.warning(
-                    "obs backend drift: expected %s here (SGP_OBS_MODE=%s%s) but the "
-                    "active span is %s; correlating against %s. Check SGP_OBS_MODE and "
-                    "the running instrumentation.",
-                    expected,
-                    get_obs_mode(),
-                    ", temporal path" if expect_otel else "",
-                    actual,
-                    actual,
-                )
+            expected, expected_probe, other_probe, actual = "ddtrace", _ddtrace_ids, _lgtm_ids, "otel"
+        if expected_probe() is not None:
+            return  # expected backend is live -> healthy; skip the other probe
+        if other_probe() is None:
+            return  # nothing live at all -> uninstrumented path, not drift
+        if (expected, actual) not in _WARNED_DRIFT:
+            _WARNED_DRIFT.add((expected, actual))
+            _log.warning(
+                "obs backend drift: expected %s here (SGP_OBS_MODE=%s%s) but the "
+                "active span is %s; correlating against %s. Check SGP_OBS_MODE and "
+                "the running instrumentation.",
+                expected,
+                get_obs_mode(),
+                ", temporal path" if expect_otel else "",
+                actual,
+                actual,
+            )
     except Exception:  # obs must never fail an app call
         pass

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

@@ -259,8 +259,9 @@ def tag_ambient_obs_span(
     Used inside the SDK's dispatched start-span/end-span activities (see
     ``trace._in_tracing_dispatch_activity``): there we must NOT open our own
     wrapper span, because start_span/end_span run as separate activities on
-    possibly different workers and the wrapper could never be closed. Instead we lean on the span the Temporal OTel ``TracingInterceptor``
-    already made active for this activity and just add
+    possibly different workers and the wrapper could never be closed. Instead we
+    lean on the span the Temporal OTel ``TracingInterceptor`` already made active
+    for this activity and just add
     ``agentex.business_span_id`` / ``agentex.business_trace_id`` so the obs -> business
     pivot still works. Best-effort; never raises.