Just a thought, but you may want to consider changing the constants defined in php/complete/constants/url_metrics_constants.php to integers instead of strings. Should make something like the following easier to use:
$cols = URLMETRICS_COL_EXTERNAL_LINKS + URLMETRICS_COL_LINKS + URLMETRICS_COL_MOZRANK;
You could have also done the below but you are correct and they should totally be integers.
$urlMetrics = array(
$cols = array_sum($urlMetrics);