$keys = array(
    "AREA" => 1
);
$initial = array(
    "show" =>0,
    "click"=>0,
    "cost"=>0,
    
);
$reduce = "function (obj,prev)
    {
        prev.show += obj.show; 
        prev.click += obj.click;
        prev.cost +=obj.cost;
    }
";

$re = $collection->group($keys, $initial, $reduce);