feat(observe): handle starlette.StreamingResponse (#1394)

Starlette has a StreamingResponse type that wraps an AsyncGenerator,
which is often used in fastapi http calls.
By wrapping the result inside of it, we can get the correct
timings and outputs for these generators in the resulting trace.

Author: yorickvP

langfuse/_client/observe.py

@@ -312,6 +312,16 @@ async def async_wrapper(*args: Tuple[Any], **kwargs: Dict[str, Any]) -> Any:
                                     transform_to_string,
                                 )
 
+                            # handle starlette.StreamingResponse
+                            if type(result).__name__ == "StreamingResponse" and hasattr(result, "body_iterator"):
+                                is_return_type_generator = True
+
+                                result.body_iterator = self._wrap_async_generator_result(
+                                    langfuse_span_or_generation,
+                                    result.body_iterator,
+                                    transform_to_string,
+                                )
+
                             langfuse_span_or_generation.update(output=result)
 
                         return result
@@ -416,6 +426,16 @@ def sync_wrapper(*args: Any, **kwargs: Any) -> Any:
                                     transform_to_string,
                                 )
 
+                            # handle starlette.StreamingResponse
+                            if type(result).__name__ == "StreamingResponse" and hasattr(result, "body_iterator"):
+                                is_return_type_generator = True
+
+                                result.body_iterator = self._wrap_async_generator_result(
+                                    langfuse_span_or_generation,
+                                    result.body_iterator,
+                                    transform_to_string,
+                                )
+
                             langfuse_span_or_generation.update(output=result)
 
                         return result