Specifies a field and a function to aggregate on.
Available functions: SUM, AVG, MIN, MAX and COUNT.
@todo What about GROUP_CONCAT support?
string $field: The name of the field to aggregate by.
string $function: The aggregation function, for example COUNT or MIN.
string $langcode: (optional) The language code.
string $alias: (optional) The key that will be used on the resultset.
\Drupal\Core\Entity\Query\QueryAggregateInterface The called object.
public function aggregate($field, $function, $langcode = NULL, &$alias = NULL);