feat(flow): 自 go-hua 迁入流量统计器

- 并发安全收发字节数累计(Add/Get/Reset), 附测试、例程与 README

flow/flow_test.go

@@ -0,0 +1,32 @@
+package flow
+
+import "testing"
+
+func TestFlowAddGet(t *testing.T) {
+	f := &Flow{}
+	f.Add(100, 200)
+	f.Add(50, 80)
+
+	send, recv := f.Get()
+	if send != 150 {
+		t.Errorf("SendFlow = %d, want 150", send)
+	}
+	if recv != 280 {
+		t.Errorf("RecvFlow = %d, want 280", recv)
+	}
+}
+
+func TestFlowReset(t *testing.T) {
+	f := &Flow{}
+	f.Add(10, 20)
+
+	send, recv := f.Reset()
+	if send != 10 || recv != 20 {
+		t.Errorf("Reset 返回 = (%d, %d), want (10, 20)", send, recv)
+	}
+
+	send, recv = f.Get()
+	if send != 0 || recv != 0 {
+		t.Errorf("Reset 后流量 = (%d, %d), want (0, 0)", send, recv)
+	}
+}