Allow configuring Micrometer Tracing MDC keys

module/spring-boot-micrometer-tracing-brave/src/main/java/org/springframework/boot/micrometer/tracing/brave/autoconfigure/BravePropagationConfigurations.java

@@ -19,6 +19,7 @@
 import java.util.List;
 
 import brave.baggage.BaggageField;
+import brave.baggage.BaggageFields;
 import brave.baggage.BaggagePropagation;
 import brave.baggage.BaggagePropagation.FactoryBuilder;
 import brave.baggage.BaggagePropagationConfig;
@@ -40,6 +41,7 @@
 import org.springframework.boot.micrometer.tracing.autoconfigure.ConditionalOnEnabledTracingExport;
 import org.springframework.boot.micrometer.tracing.autoconfigure.TracingProperties;
 import org.springframework.boot.micrometer.tracing.autoconfigure.TracingProperties.Baggage.Correlation;
+import org.springframework.boot.micrometer.tracing.autoconfigure.TracingProperties.Mdc;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.annotation.Order;
@@ -138,7 +140,15 @@ Factory propagationFactory(BaggagePropagation.FactoryBuilder factoryBuilder) {
 		@ConditionalOnMissingBean
 		CorrelationScopeDecorator.Builder mdcCorrelationScopeDecoratorBuilder(
 				ObjectProvider<CorrelationScopeCustomizer> correlationScopeCustomizers) {
-			CorrelationScopeDecorator.Builder builder = MDCScopeDecorator.newBuilder();
+			Mdc mdc = this.tracingProperties.getMdc();
+			CorrelationScopeDecorator.Builder builder = MDCScopeDecorator.newBuilder()
+				// Clear existing traceId/spanId backage field mappings
+				// so the MDC key names can be customized below.
+				// BravePropagationConfigurationsTests validates the assumption that
+				// the builder only configures the trace/span id by default.
+				.clear()
+				.add(SingleCorrelationField.newBuilder(BaggageFields.TRACE_ID).name(mdc.getTraceIdKey()).build())
+				.add(SingleCorrelationField.newBuilder(BaggageFields.SPAN_ID).name(mdc.getSpanIdKey()).build());
 			correlationScopeCustomizers.orderedStream().forEach((customizer) -> customizer.customize(builder));
 			return builder;
 		}

module/spring-boot-micrometer-tracing-brave/src/test/java/org/springframework/boot/micrometer/tracing/brave/autoconfigure/BraveAutoConfigurationTests.java

@@ -258,6 +258,31 @@ void shouldSupplyMdcCorrelationScopeDecoratorIfBaggageCorrelationDisabled() {
 			.run((context) -> assertThat(context).hasBean("mdcCorrelationScopeDecoratorBuilder"));
 	}
 
+	@Test
+	void correlationScopeDecoratorUsesDefaultMdcKeys() {
+		this.contextRunner.run((context) -> {
+			ScopeDecorator scopeDecorator = context.getBean(ScopeDecorator.class);
+			assertThat(scopeDecorator)
+				.extracting("fields", InstanceOfAssertFactories.array(SingleCorrelationField[].class))
+				.extracting(SingleCorrelationField::name)
+				.containsExactly("traceId", "spanId");
+		});
+	}
+
+	@Test
+	void correlationScopeDecoratorUsesCustomMdcKeys() {
+		this.contextRunner
+			.withPropertyValues("management.tracing.mdc.trace-id-key=customTraceId",
+					"management.tracing.mdc.span-id-key=customSpanId")
+			.run((context) -> {
+				ScopeDecorator scopeDecorator = context.getBean(ScopeDecorator.class);
+				assertThat(scopeDecorator)
+					.extracting("fields", InstanceOfAssertFactories.array(SingleCorrelationField[].class))
+					.extracting(SingleCorrelationField::name)
+					.containsExactly("customTraceId", "customSpanId");
+			});
+	}
+
 	@Test
 	void shouldHave128BitTraceId() {
 		this.contextRunner.run((context) -> {

module/spring-boot-micrometer-tracing-brave/src/test/java/org/springframework/boot/micrometer/tracing/brave/autoconfigure/BravePropagationConfigurationsTests.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2012-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.micrometer.tracing.brave.autoconfigure;
+
+import brave.baggage.BaggageFields;
+import brave.baggage.CorrelationScopeConfig.SingleCorrelationField;
+import brave.baggage.CorrelationScopeDecorator;
+import brave.context.slf4j.MDCScopeDecorator;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link BravePropagationConfigurations}.
+ */
+class BravePropagationConfigurationsTests {
+
+	/**
+	 * Validates the assumption that {@link MDCScopeDecorator#newBuilder()} only
+	 * configures traceId and spanId by default. The
+	 * {@link BravePropagationConfigurations.PropagationWithBaggage#mdcCorrelationScopeDecoratorBuilder}
+	 * method clears the builder's defaults and re-adds only these two fields so their MDC
+	 * key names can be customized. If Brave adds new default fields in the future, the
+	 * {@code .clear()} call in that method would silently drop them, and this test will
+	 * catch that.
+	 */
+	@Test
+	void mdcScopeDecoratorBuilderShouldOnlyHaveTraceIdAndSpanIdByDefault() {
+		CorrelationScopeDecorator.Builder builder = MDCScopeDecorator.newBuilder();
+		assertThat(builder.configs()).extracting((config) -> ((SingleCorrelationField) config).name())
+			.containsExactlyInAnyOrder(BaggageFields.TRACE_ID.name(), BaggageFields.SPAN_ID.name());
+	}
+
+}

module/spring-boot-micrometer-tracing-opentelemetry/src/main/java/org/springframework/boot/micrometer/tracing/opentelemetry/autoconfigure/OpenTelemetryTracingAutoConfiguration.java

@@ -59,6 +59,7 @@
 import org.springframework.boot.micrometer.tracing.autoconfigure.MicrometerTracingAutoConfiguration;
 import org.springframework.boot.micrometer.tracing.autoconfigure.NoopTracerAutoConfiguration;
 import org.springframework.boot.micrometer.tracing.autoconfigure.TracingProperties;
+import org.springframework.boot.micrometer.tracing.autoconfigure.TracingProperties.Mdc;
 import org.springframework.boot.micrometer.tracing.opentelemetry.autoconfigure.OpenTelemetryPropagationConfigurations.NoPropagation;
 import org.springframework.boot.micrometer.tracing.opentelemetry.autoconfigure.OpenTelemetryPropagationConfigurations.PropagationWithBaggage;
 import org.springframework.boot.micrometer.tracing.opentelemetry.autoconfigure.OpenTelemetryPropagationConfigurations.PropagationWithoutBaggage;
@@ -219,7 +220,8 @@ OtelCurrentTraceContext otelCurrentTraceContext() {
 	@Bean
 	@ConditionalOnMissingBean
 	Slf4JEventListener otelSlf4JEventListener() {
-		return new Slf4JEventListener();
+		Mdc mdc = this.tracingProperties.getMdc();
+		return new Slf4JEventListener(mdc.getTraceIdKey(), mdc.getSpanIdKey());
 	}
 
 	@Bean

module/spring-boot-micrometer-tracing-opentelemetry/src/test/java/org/springframework/boot/micrometer/tracing/opentelemetry/autoconfigure/OpenTelemetryTracingAutoConfigurationTests.java

@@ -190,6 +190,27 @@ void samplerCanBeSetToParentBasedTraceIdRatio() {
 			});
 	}
 
+	@Test
+	void slf4jEventListenerUsesDefaultMdcKeys() {
+		this.contextRunner.run((context) -> {
+			Slf4JEventListener listener = context.getBean(Slf4JEventListener.class);
+			assertThat(listener).hasFieldOrPropertyWithValue("traceIdKey", "traceId")
+				.hasFieldOrPropertyWithValue("spanIdKey", "spanId");
+		});
+	}
+
+	@Test
+	void slf4jEventListenerUsesCustomMdcKeys() {
+		this.contextRunner
+			.withPropertyValues("management.tracing.mdc.trace-id-key=customTraceId",
+					"management.tracing.mdc.span-id-key=customSpanId")
+			.run((context) -> {
+				Slf4JEventListener listener = context.getBean(Slf4JEventListener.class);
+				assertThat(listener).hasFieldOrPropertyWithValue("traceIdKey", "customTraceId")
+					.hasFieldOrPropertyWithValue("spanIdKey", "customSpanId");
+			});
+	}
+
 	@ParameterizedTest
 	@ValueSource(strings = { "io.micrometer.tracing.otel", "io.opentelemetry.sdk", "io.opentelemetry.api" })
 	void shouldNotSupplyBeansIfDependencyIsMissing(String packageName) {

module/spring-boot-micrometer-tracing/src/main/java/org/springframework/boot/micrometer/tracing/autoconfigure/TracingProperties.java

@@ -53,6 +53,11 @@ public class TracingProperties {
 	 */
 	private final Exemplars exemplars = new Exemplars();
 
+	/**
+	 * {@code org.slf4j.MDC} key configuration for trace and span IDs.
+	 */
+	private final Mdc mdc = new Mdc();
+
 	public Sampling getSampling() {
 		return this.sampling;
 	}
@@ -69,6 +74,10 @@ public Exemplars getExemplars() {
 		return this.exemplars;
 	}
 
+	public Mdc getMdc() {
+		return this.mdc;
+	}
+
 	public static class Sampling {
 
 		/**
@@ -258,6 +267,39 @@ public enum PropagationType {
 
 	}
 
+	/**
+	 * {@code org.slf4j.MDC} key configuration for trace and span IDs.
+	 */
+	public static class Mdc {
+
+		/**
+		 * Key under which the trace ID is added to the {@code org.slf4j.MDC}.
+		 */
+		private String traceIdKey = "traceId";
+
+		/**
+		 * Key under which the span ID is added to the {@code org.slf4j.MDC}.
+		 */
+		private String spanIdKey = "spanId";
+
+		public String getTraceIdKey() {
+			return this.traceIdKey;
+		}
+
+		public void setTraceIdKey(String traceIdKey) {
+			this.traceIdKey = traceIdKey;
+		}
+
+		public String getSpanIdKey() {
+			return this.spanIdKey;
+		}
+
+		public void setSpanIdKey(String spanIdKey) {
+			this.spanIdKey = spanIdKey;
+		}
+
+	}
+
 	/**
 	 * Exemplars configuration.
 	 */