currently you can’t use query with aggregate functions in developer’s REST CONSOLE
Hi Yuriy,
You’re right, we’re aware of it and internally also lack this feature while testing things 
Unfortunately, we hadn’t time to put it to the roadmap yet, but I will try to raise the discussion once more.
As a workaround (which I personally use as well for now), you can export the request as curl, copy the URL from it to the browser’s address bar in a new tab and add the required aggregated properties to the props query option.
Thanks for the suggestion!