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.

src/polygateway/ports.py

@@ -274,4 +274,5 @@ class TelemetryRecorder(Protocol):
         cost: float | None,
         cached_prompt_tokens: int | None,
         model_reported: str | None,
+        sampling: str | None,
     ) -> None: ...