code: def insert(self, key, value):
          
          if key in self.map:
              return
  
          try:
              tag_key = TagKey(key)
              tag_val = TagValue(value)
              self.map[tag_key] = tag_val
          except ValueError:
              raise
docs: Inserts a key and value in the map if the map does not already
          contain the key.
  
          :type key: :class: '~opencensus.tags.tag_key.TagKey'
          :param key: a tag key to insert into the map
  
          :type value: :class: '~opencensus.tags.tag_value.TagValue'
          :param value: a tag value that is associated with the tag key and
          the value to insert into the tag map