getSingle* APIs: improvements for 'orderby' argument

[nikmagini]

1. The 'orderby' argument is misnamed in getSingle* APIs e.g. getSingleDSStat - it doesn't actually change the ordering of the results, it just changes which of the three popularity metrics (totcpu, nacc, nusers) is returned in the output. 'metric' would be a more appropriate name

2. For this reason, it makes sense to support passing multiple values to this argument, to return any/all metrics in output