src/Models/Meta.php
public function addExtra($key, $value)
    {
        return $this->setExtras(
            $this->extras->put($key, $value)->all()
        );
    }
public function addExtra($key, $value)
    {
        return $this->setExtras(
            $this->extras->put($key, $value)->all()
        );
    }
Add an extra meta.

@param  string  $key
@param  mixed   $value

@return $this