code: func (c gracefulConn) Close() error {
  	err := c.Conn.Close()
  	if err != nil {
  		return err
  	}
  	// close can fail on http2 connections (as of Oct. 2015, before http2 in std lib)
  	// so don't decrement count unless close succeeds
  	c.connWg.Done()
  	return nil
  }
docs: // Close closes c's underlying connection while updating the wg count.