Updates the map by updating the value of a key

        :type key: :class: '~opencensus.tags.tag_key.TagKey'
        :param key: A tag key to be updated

        :type value: :class: '~opencensus.tags.tag_value.TagValue'
        :param value: The value to update the key to in the map

def update(self, key, value):
        """Updates the map by updating the value of a key

        :type key: :class: '~opencensus.tags.tag_key.TagKey'
        :param key: A tag key to be updated

        :type value: :class: '~opencensus.tags.tag_value.TagValue'
        :param value: The value to update the key to in the map

        """
        if key in self.map:
            self.map[key] = value