During SERVER-84314 we found that this is probably a possible race. Something like the following:

T1: gets request, inserts query stats entry, calls counter.increment().
T2: gets request, clears query stats store, evicting that entry. calls counter.resetToZero(), which then applies the atomic operation. _counter is now 0.
T1: applies the atomic operation _counter.fetchAndAdd(1). _counter is now 1, even though the entry got evicted.

I split it out into a follow-up ticket since this might be more widespread than just the query stats store, and also it's unclear how much we really care about it. I think the worst case scenario is that we end up out of sync between our serverStatus metrics and the actual truth of the size.