fix race condition

Author: YaroShkvorets

db/db.go

@@ -454,8 +454,11 @@ func (l *Loader) HasTable(tableName string) bool {
 }
 
 // GetActiveFlushesNum returns the current number of in-flight async flushes.
-// This value is approximate and may be racy; it is intended for metrics only.
-func (l *Loader) GetActiveFlushesNum() int { return l.activeFlushes }
+func (l *Loader) GetActiveFlushesNum() int {
+	l.cond.L.Lock()
+	defer l.cond.L.Unlock()
+	return l.activeFlushes
+}
 
 func (l *Loader) MarshalLogObject(encoder zapcore.ObjectEncoder) error {
 	encoder.AddUint64("entries_count", l.entriesCount)