fix: "metadata" issue

backend/apps/openai/main.py

@@ -356,7 +356,9 @@ async def generate_chat_completion(
 ):
     idx = 0
     payload = {**form_data}
-    payload.pop("metadata")
+
+    if "metadata" in payload:
+        payload.pop("metadata")
 
     model_id = form_data.get("model")
     model_info = Models.get_model_by_id(model_id)