code: func (s *Client) NewSubStatter(prefix string) SubStatter {
  	var c *Client
  	if s != nil {
  		c = &Client{
  			prefix:  joinPathComp(s.prefix, prefix),
  			sender:  s.sender,
  			sampler: s.sampler,
  		}
  	}
  	return c
  }
docs: // NewSubStatter returns a SubStatter with appended prefix