fix reference

Author: crimson-gao

instrumentation-genai/opentelemetry-instrumentation-mcp/src/opentelemetry/instrumentation/mcp/session_handler.py

@@ -251,13 +251,14 @@ def extract_attributes(self, args: Tuple[Any, ...]) -> Tuple[Dict[str, str], Uni
 
             # span name
             if method in ["tools/call", "prompts/get"]:
-                target_name : str = message.request.root.params.name # type: ignore
+                target_name = message.request.root.params.name # type: ignore
                 span_name = f"{method} {target_name}"
             elif method in ["resources/read", "resources/subscribe", "resources/unsubscribe"]:
                 target_name = str(message.request.root.params.uri) # type: ignore
                 span_name = f"{method} {target_name}"
             else:
                 span_name = method
+                target_name = None
 
             if target_name is not None and method in _method_names_with_target:
                 (attr_name, _) = _method_names_with_target[method]