use standard context manager

Author: daniel-sanche

google/cloud/bigtable/data/_metrics/data_model.py

@@ -409,19 +409,19 @@ def _handle_error(message: str) -> None:
         full_message = f"Error in Bigtable Metrics: {message}"
         LOGGER.warning(full_message)
 
-    async def __aenter__(self):
+    def __aenter__(self):
         """
-        Implements the async context manager protocol for wrapping unary calls
+        Implements the async manager protocol
 
         Using the operation's context manager provides assurances that the operation
         is always closed when complete, with the proper status code automaticallty
         detected when an exception is raised.
         """
         return self
 
-    async def __aexit__(self, exc_type, exc_val, exc_tb):
+    def __aexit__(self, exc_type, exc_val, exc_tb):
         """
-        Implements the async context manager protocol for wrapping unary calls
+        Implements the context manager protocol
 
         The operation is automatically ended on exit, with the status determined
         by the exception type and value.