The spec states that this should return an iterable. If we have no plans of distinct changing in the future (e.g. returning a cursor), then we can stick to an array return type (as with the current driver).

Usage example:

$collection->distinct(
    // Required field param
    'y',
    // Optional named params in an associative array (BC break, criteria in 1.x)
    [
        'criteria' => ['x' => 1],
        'maxTimeMS' => 2000,
    ]
);