feat(go): expose gateway connection close metrics

go/vehicle-gateway/internal/gateway/tcp_server.go

@@ -146,6 +146,7 @@ func (s *TCPServer) ListenAndServe(ctx context.Context) error {
 			}()
 		default:
 			s.recordConnectionRejection("max_connections")
+			s.recordConnectionClose("max_connections")
 			s.logger.Warn("tcp connection rejected: max connections reached", "protocol", s.protocol.Protocol, "remote", conn.RemoteAddr().String())
 			_ = conn.Close()
 		}
@@ -172,6 +173,7 @@ func (s *TCPServer) handleConnection(ctx context.Context, conn net.Conn) {
 			frames, remainder, extractErr := s.protocol.Extract(pending)
 			if extractErr != nil {
 				log.Warn("frame extraction failed", "error", extractErr)
+				s.recordConnectionClose("extract_error")
 				return
 			}
 			pending = remainder
@@ -181,17 +183,21 @@ func (s *TCPServer) handleConnection(ctx context.Context, conn net.Conn) {
 		}
 		if err != nil {
 			if errors.Is(err, io.EOF) {
+				s.recordConnectionClose("eof")
 				return
 			}
 			var netErr net.Error
 			if errors.As(err, &netErr) && netErr.Timeout() {
 				log.Warn("tcp connection idle timeout")
+				s.recordConnectionClose("read_timeout")
 				return
 			}
 			log.Warn("tcp read failed", "error", err)
+			s.recordConnectionClose("read_error")
 			return
 		}
 		if ctx.Err() != nil {
+			s.recordConnectionClose("context_cancelled")
 			return
 		}
 	}
@@ -386,6 +392,16 @@ func (s *TCPServer) recordConnectionRejection(reason string) {
 	})
 }
 
+func (s *TCPServer) recordConnectionClose(reason string) {
+	if s.metrics == nil {
+		return
+	}
+	s.metrics.IncCounter("vehicle_gateway_connection_closes_total", metrics.Labels{
+		"protocol": string(s.protocol.Protocol),
+		"reason":   reason,
+	})
+}
+
 func identityStatus(env envelope.FrameEnvelope) string {
 	if strings.TrimSpace(env.VIN) != "" {
 		return "resolved"

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