Use Self type for context manager in telemetry.py

Author: vinoo999

src/a2a/utils/telemetry.py

@@ -71,6 +71,7 @@ def internal_method(self):
 
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Any
+from typing_extensions import Self
 
 
 if TYPE_CHECKING:
@@ -119,7 +120,7 @@ class _NoOp:
         def __call__(self, *args: Any, **kwargs: Any) -> Any:
             return self
 
-        def __enter__(self) -> '_NoOp':
+        def __enter__(self) -> Self:
             return self
 
         def __exit__(self, *args: object, **kwargs: Any) -> None: