code: func SetSnifferCallback(f SnifferCallback) ClientOptionFunc {
  	return func(c *Client) error {
  		if f != nil {
  			c.snifferCallback = f
  		}
  		return nil
  	}
  }
docs: // SetSnifferCallback allows the caller to modify sniffer decisions.
  // When setting the callback, the given SnifferCallback is called for
  // each (healthy) node found during the sniffing process.
  // If the callback returns false, the node is ignored: No requests
  // are routed to it.