src/Sender/Builder/Http20Builder.php
public function addVisitor(HttpProtocolVisitorInterface $visitor, int $priority = 0): Http20Builder
    {
        $this->visitors->insert($visitor, $priority);

        return $this;
    }
public function addVisitor(HttpProtocolVisitorInterface $visitor, int $priority = 0): Http20Builder
    {
        $this->visitors->insert($visitor, $priority);

        return $this;
    }
Add visitor

@param HttpProtocolVisitorInterface $visitor
@param int                          $priority

@return Http20Builder