Old-style check counters aren't getting cleared

[conorbranagan]

If you sample a counter at ts1 and ts2, the counter will continue to report for every check loop as the same value at ts2.

The correct behavior is we should clear all values except the last for a counter on each flush. Therefore if no values come in during the next check run, no value will be flushed. But if a new value does come in, the rate will still be calculated against the last one.