feat(go): expose gateway connection close metrics

go/vehicle-gateway/internal/gateway/tcp_server_test.go

@@ -164,6 +164,30 @@ func TestTCPServerRecordsConnectionRejectionMetric(t *testing.T) {
 	}
 }
 
+func TestTCPServerRecordsConnectionCloseReasonMetric(t *testing.T) {
+	registry := metrics.NewRegistry()
+	server, err := NewTCPServer(TCPServerConfig{
+		Protocol: TCPProtocol{
+			Protocol: envelope.ProtocolJT808,
+			Addr:     ":0",
+			Extract:  jt808.ExtractFrames,
+			Parse:    jt808.ParseFrame,
+		},
+		Sink:    &recordingSink{},
+		Logger:  slog.New(slog.NewTextHandler(testWriter{t: t}, nil)),
+		Metrics: registry,
+	})
+	if err != nil {
+		t.Fatalf("NewTCPServer() error = %v", err)
+	}
+
+	server.recordConnectionClose("read_timeout")
+
+	if text := registry.Render(); !strings.Contains(text, `vehicle_gateway_connection_closes_total{protocol="JT808",reason="read_timeout"} 1`) {
+		t.Fatalf("connection close metric missing:\n%s", text)
+	}
+}
+
 func TestTCPServerPublishesBadFrameOnlyToRaw(t *testing.T) {
 	good := buildGBFrame(0x02, 0xfe, "LNBSCB3D4R1234567", nil)
 	good[len(good)-1] ^= 0xff