feat: record sampling parameters in telemetry (port 20 to 21 fields)

Each of the three emitter entry points has a pinned meaning: only the
attempt path has an effective source, so only it merges extra_body.

tests/integration/test_postgres_telemetry.py

@@ -41,6 +41,7 @@ _EXPECTED_COLUMNS = [
     "created_at",
     "cached_prompt_tokens",
     "model_reported",
+    "sampling",
 ]
 
 # run 级前缀: 同库并存的其他运行(迁移批跑/另一开发机)互不可见
@@ -104,6 +105,7 @@ async def _record_minimal(
         "cost": None,
         "cached_prompt_tokens": None,
         "model_reported": None,
+        "sampling": None,
     }
     fields.update(overrides)
     await recorder.record_llm_call(**fields)