Allow configuring Micrometer Tracing MDC keys

See gh-50595

Signed-off-by: Phil Clay <philsttr@users.noreply.github.com>

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

@@ -19,6 +19,7 @@ package org.springframework.boot.micrometer.tracing.brave.autoconfigure;
 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.context.properties.EnableConfigurationProperties
 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 @@ class BravePropagationConfigurations {
 		@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 @@ class BraveAutoConfigurationTests {
 			.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());
+	}
+
+}