code: def update(self, key, value):
          
          if key in self.map:
              self.map[key] = value
docs: 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