Get aggregate list of all custom fields for entire blog

The function below will store a distinct list of all custom fields for the list of posts/pages/custom posts that is passed to it in the keys of the array $customfields. The array values are the number of posts with the corresponding field. In the example, custom fields added by plugins are excluded ($value[0] != '_';) but these could easily be added back in.

function all_custom_fields($allposts) {
    foreach ( $allposts as $post ) : setup_postdata($post);
        $post_id = $post->ID;
        $fields = get_post_custom_keys($post_id);    // all keys for post as values of array
        if ($fields) {
            foreach ($fields as $key => $value) {
                if ($value[0] != '_') {              // exclude where added by plugin
                    $customfields[$value] = isset($customfields[$value]) ? $customfields[$value] + 1 : 1;
                }
            }
        }
    endforeach; wp_reset_postdata();
    return $customfields;
}

// example - all post types, whether published or not
$args = array(
    'post_status' => array('publish','draft','pending','future'),
    'post_type' => 'any',
    'posts_per_page' => -1,
);
$allposts = get_posts($args);
$customfields = all_custom_fields($allposts);