Turn off all references and API calls to billing if the feature is disabled

endpoints/api/__init__.py

@@ -124,6 +124,9 @@ def format_date(date):
 
 def add_method_metadata(name, value):
   def modifier(func):
+    if func is None:
+      return None
+
     if '__api_metadata' not in dir(func):
       func.__api_metadata = {}
     func.__api_metadata[name] = value
@@ -132,6 +135,9 @@ def add_method_metadata(name, value):
 
 
 def method_metadata(func, name):
+  if func is None:
+    return None
+
   if '__api_metadata' in dir(func):
     return func.__api_metadata.get(name, None)
   return None