statsitics: clear the fmsketch's map correctly (#49379)

close #49381

pkg/statistics/fmsketch.go

@@ -217,16 +217,7 @@ func (s *FMSketch) MemoryUsage() (sum int64) {
 }
 
 func (s *FMSketch) reset() {
-	// not use hashset.Clear, it will release all memory and Not conducive to memory reuse.
-	// the size of set is not more than 10000.
-	set := make([]uint64, 0, s.hashset.Count())
-	s.hashset.Iter(func(k uint64, v bool) (stop bool) {
-		set = append(set, k)
-		return false
-	})
-	for _, k := range set {
-		s.hashset.Delete(k)
-	}
+	s.hashset.Clear()
 	s.mask = 0
 	s.maxSize = 0
 }