AssetMetadataListener.php

<?php

namespace AppBundle\EventListener;

use Pimcore\Event\Model\AssetEvent;

class AssetMetadataListener
{
    public function onAssetCreate(AssetEvent $event)
    {
        $asset = $event->getAsset();

        $asset->addMetadata('awesomeBoolean', 'checkbox', true);
        $asset->addMetadata('someString', 'input', uniqid());

        // ...
        // ...
    }
}

services.yml

    AppBundle\EventListener\AssetMetadataListener:
         tags:
             - { name: kernel.event_listener, event: pimcore.asset.preAdd, method: onAssetCreate }