Copy the free pages stats from the receiver

db.go

@@ -579,6 +579,10 @@ func (s *Stats) Sub(other *Stats) Stats {
 		return *s
 	}
 	var diff Stats
+	diff.FreePageN = s.FreePageN
+	diff.PendingPageN = s.PendingPageN
+	diff.FreeAlloc = s.FreeAlloc
+	diff.FreelistInuse = s.FreelistInuse
 	diff.TxN = other.TxN - s.TxN
 	diff.TxStats = s.TxStats.Sub(&other.TxStats)
 	return diff

db_test.go

@@ -354,9 +354,13 @@ func TestDB_String(t *testing.T) {
 func TestDBStats_Sub(t *testing.T) {
 	var a, b Stats
 	a.TxStats.PageCount = 3
+	a.FreePageN = 4
 	b.TxStats.PageCount = 10
+	b.FreePageN = 14
 	diff := b.Sub(&a)
 	assert.Equal(t, 7, diff.TxStats.PageCount)
+	// free page stats are copied from the receiver and not subtracted
+	assert.Equal(t, 14, diff.FreePageN)
 }
 
 // Ensure that meta with bad magic is invalid.