fs: fix goroutine leak and improve stats accounting process

This fixes the go routine leak in the stats accounting

- don't start stats average loop when initializing `StatsInfo`
- stop the loop instead of pausing
- use a context instead of a channel
- move `period` variable in `averageValues` struct

Fixes #8570

fs/accounting/stats_groups.go

@@ -298,6 +298,7 @@ func GlobalStats() *StatsInfo {
 // NewStatsGroup creates new stats under named group.
 func NewStatsGroup(ctx context.Context, group string) *StatsInfo {
 	stats := NewStats(ctx)
+	stats.startAverageLoop()
 	stats.group = group
 	groups.set(ctx, group, stats)
 	return stats