ResetCounters unconditionally restarted the average loop, spawning a
ticker goroutine that pinned the StatsInfo even when no loop had been
running before. statsGroups.delete calls ResetCounters on every removed
group, so deleting N stats groups leaked N goroutines and prevented GC
of the underlying StatsInfo objects.
Only restart the loop if it was active before the reset.
Nick Craig-Wood
a8f102ce8f