use module level cache

Author: ohmayr

gapic/schema/api.py

@@ -1230,6 +1230,7 @@ def proto(self) -> Proto:
 
         global_collisions = frozenset(naive.names)
         visited_messages: Set[wrappers.MessageType] = set()
+        self.context_cache = {}
         # Return a context-aware proto object.
         return dataclasses.replace(
             naive,