/**
* Returns the average value of the numeric values that result from applying
* a specified expression to each document in a group of documents that
* share the same group by key. Ignores nun-numeric values.
*
* @see Expr::avg
* @param mixed|Expr $expression
* @return Operator
*/
public function avg($expression)
{
$this->expr->avg($expression);
return $this;
}
