def GetRequestClass(self, method_descriptor):
raise NotImplementedError<EOL>
Returns the class of the request message for the specified method.

        CallMethod() requires that the request is of a particular subclass of
        Message. GetRequestClass() gets the default instance of this required
        type.

        Example:
          method = service.GetDescriptor().FindMethodByName("Foo")
          request = stub.GetRequestClass(method)()
          request.ParseFromString(input)
          service.CallMethod(method, request, callback)