Custom dimensions are not tracked in Google Analytics HTTP requests (measurement protocol)

When using analytics.js, I can successfully track events including custom dimensions this way (as described in the docs ):

ga('send', 'event', 'category', 'action', {
    'metric18': 8000,
    'dimension6': 'crocodile'
});

      

        
        
        
      

    

However, when using the measurement protocol (i.e. HTTP requests), I cannot find a way to include custom dimensions and metrics in event tracking, since I did not find a reference in the documentation.

This is what I have tried so far (based on examples found in the documentation ). In both cases, the event was indeed tracked, but without any custom dimensions or associated metrics.

http://www.google-analytics.com/collect?
 v=1             // Version.
 &tid=UA-XXXX-Y  // Tracking ID / Property ID.
 &cid=555        // Anonymous Client ID.

 &t=event        // Event hit type
 &ec=video       // Event Category. Required.
 &ea=play        // Event Action. Required.
 &metric18=8000
 &dimension6=crocodile

      

        
        
        
      

    

and

http://www.google-analytics.com/collect?
 v=1             // Version.
 &tid=UA-XXXX-Y  // Tracking ID / Property ID.
 &cid=555        // Anonymous Client ID.

 &t=event        // Event hit type
 &ec=video       // Event Category. Required.
 &ea=play        // Event Action. Required.
 &el={"metric18":8000,"dimension6":"crocodile"}