fix: use frame protocol for metrics timing

modules/core/observability/src/main/java/com/lingniu/ingest/observability/IngestMetrics.java

@@ -63,7 +63,7 @@ public class IngestMetrics implements IngestInterceptor, Ordered {
 
         Timer.Sample s = ctx.attr(ATTR_TIMER_SAMPLE);
         if (s != null) {
-            stopTimer(ctx, event.source() == null ? "unknown" : event.source().name());
+            stopTimer(ctx, protocolTag(ctx));
         }
     }
 

modules/core/observability/src/test/java/com/lingniu/ingest/observability/IngestMetricsTest.java

@@ -1,6 +1,7 @@
 package com.lingniu.ingest.observability;
 
 import com.lingniu.ingest.api.ProtocolId;
+import com.lingniu.ingest.api.event.VehicleEvent;
 import com.lingniu.ingest.api.pipeline.IngestContext;
 import com.lingniu.ingest.api.pipeline.RawFrame;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
@@ -41,6 +42,28 @@ class IngestMetricsTest {
                 .isEqualTo(1);
     }
 
+    @Test
+    void recordsFrameDurationAgainstTheFrameProtocolCapturedInBefore() {
+        SimpleMeterRegistry registry = new SimpleMeterRegistry();
+        IngestMetrics metrics = new IngestMetrics(registry);
+        IngestContext context = new IngestContext("trace-4");
+
+        metrics.before(frame(ProtocolId.JT808), context);
+        metrics.after(new VehicleEvent.Heartbeat(
+                "event-1",
+                "VIN001",
+                ProtocolId.UNKNOWN,
+                Instant.parse("2026-07-01T00:00:01Z"),
+                Instant.parse("2026-07-01T00:00:02Z"),
+                "trace-4",
+                Map.of()), context);
+
+        assertThat(registry.timer("ingest_frame_duration_seconds", "protocol", "JT808").count())
+                .isEqualTo(1);
+        assertThat(registry.find("ingest_frame_duration_seconds").tag("protocol", "unknown").timer())
+                .isNull();
+    }
+
     @Test
     void countsErrorsAsUnknownWhenThereIsNoFrameProtocolContext() {
         SimpleMeterRegistry registry = new SimpleMeterRegistry();