fix: annotate telemetry __enter__ with Self

Author: Wondr-design

src/a2a/utils/telemetry.py

@@ -62,6 +62,12 @@ def internal_method(self):
 from typing import TYPE_CHECKING, Any
 
 
+try:
+    from typing import Self
+except ImportError:  # pragma: no cover - for Python < 3.11
+    from typing_extensions import Self
+
+
 if TYPE_CHECKING:
     from opentelemetry.trace import SpanKind as SpanKindType
 else:
@@ -86,7 +92,7 @@ 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: