Give Prometheus exemplar trace_id and span_id precedence over filtered attributes (#8959)

Author: thswlsqls

exporters/prometheus/src/main/java/io/opentelemetry/exporter/prometheus/Otel2PrometheusConverter.java

@@ -540,9 +540,9 @@ private Labels convertAttributes(
 
     Map<String, String> labelNameToValue = buildAttributeLabels(attributes);
 
+    // Additional attributes (exemplar trace_id and span_id) take precedence over attributes.
     for (int i = 0; i < additionalAttributes.length; i += 2) {
-      labelNameToValue.putIfAbsent(
-          requireNonNull(additionalAttributes[i]), additionalAttributes[i + 1]);
+      labelNameToValue.put(requireNonNull(additionalAttributes[i]), additionalAttributes[i + 1]);
     }
 
     if (scope != null && otelScopeLabelsEnabled) {

exporters/prometheus/src/test/java/io/opentelemetry/exporter/prometheus/Otel2PrometheusConverterTest.java

@@ -1408,6 +1408,48 @@ private static Stream<Arguments> exemplarLabelLimitArgs() {
             new String[] {}));
   }
 
+  @ParameterizedTest
+  @MethodSource("exemplarSpanContextLabelsTakePrecedenceArgs")
+  void exemplarSpanContextLabelsTakePrecedence(Attributes filteredAttributes) {
+    SpanContext spanContext =
+        SpanContext.create(
+            "00000000000000000000000000000001",
+            "0000000000000001",
+            TraceFlags.getSampled(),
+            TraceState.getDefault());
+    ImmutableDoubleExemplarData exemplar =
+        (ImmutableDoubleExemplarData)
+            ImmutableDoubleExemplarData.create(filteredAttributes, 1000L, spanContext, 1.0);
+
+    MetricData metricData =
+        ImmutableMetricData.createDoubleGauge(
+            Resource.getDefault(),
+            InstrumentationScopeInfo.create("test"),
+            "my.gauge",
+            "desc",
+            "unit",
+            ImmutableGaugeData.create(
+                Collections.singletonList(
+                    ImmutableDoublePointData.create(
+                        0, 1000, Attributes.empty(), 1.0, Collections.singletonList(exemplar)))));
+
+    MetricSnapshots snapshots = converter.convert(Collections.singletonList(metricData));
+    GaugeDataPointSnapshot point = (GaugeDataPointSnapshot) snapshots.get(0).getDataPoints().get(0);
+    Labels exemplarLabels = point.getExemplar().getLabels();
+    assertThat(exemplarLabels.get("trace_id")).isEqualTo("00000000000000000000000000000001");
+    assertThat(exemplarLabels.get("span_id")).isEqualTo("0000000000000001");
+  }
+
+  private static Stream<Arguments> exemplarSpanContextLabelsTakePrecedenceArgs() {
+    return Stream.of(
+        Arguments.argumentSet(
+            "sameLabelName",
+            Attributes.of(stringKey("trace_id"), "bogus", stringKey("span_id"), "bogus")),
+        Arguments.argumentSet(
+            "sameLabelNameAfterEscaping",
+            Attributes.of(stringKey("trace.id"), "bogus", stringKey("span.id"), "bogus")));
+  }
+
   private static InfoSnapshot makeInfoSnapshot(String id) {
     return new InfoSnapshot(
         MetricMetadata.builder().name("target").build(),