Add API function to stop flushDaemon

The new API function stops the daemons running goroutine. This way,
users can gracefully shut down klog and prevent goroutine leakage.

klog.go

@@ -1354,6 +1354,13 @@ func (f *flushDaemon) isRunning() bool {
 	return f.stopC != nil
 }
 
+// StopFlushDaemon stops the flush daemon, if running.
+// This prevents klog from leaking goroutines on shutdown. After stopping
+// the daemon, you can still manually flush buffers by calling Flush().
+func StopFlushDaemon() {
+	logging.flushD.stop()
+}
+
 // lockAndFlushAll is like flushAll but locks l.mu first.
 func (l *loggingT) lockAndFlushAll() {
 	l.mu.Lock()

klog_test.go

@@ -2005,3 +2005,16 @@ func TestFlushDaemon(t *testing.T) {
 		}
 	}
 }
+
+func TestStopFlushDaemon(t *testing.T) {
+	logging.flushD.stop()
+	logging.flushD = newFlushDaemon(time.Second, func() {})
+	logging.flushD.run()
+	if !logging.flushD.isRunning() {
+		t.Error("expected flushD to be running")
+	}
+	StopFlushDaemon()
+	if logging.flushD.isRunning() {
+		t.Error("expected flushD to be stopped")
+	}
+}