@instance_stats_calculator(per_second=True)<EOL>def delta_ps(prev, curr, counters):
counter = get_counter(counters)<EOL>pv = getattr(prev, counter)<EOL>cv = getattr(curr, counter)<EOL>return minus(cv, pv)<EOL>
calculate the delta per second of one counter

    formula: (curr - prev) / delta_time
    :param prev: previous resource
    :param curr: current resource
    :param counters: the counter to do delta and per second, one only
    :return: value, NaN if invalid.