Set instance property to a value and add it varz if needed

def _set_instance_prop(self, attr_name, config_prop, value):
        """Set instance property to a value and add it varz if needed"""
        setattr(self, attr_name, value)

        # add to varz if it is not private
        if not config_prop.exclude_from_varz:
            self.varz[attr_name] = value