Add missing BaggageTextMapPropagator for OTel W3C

Without this change we're missing the BaggageTextMapPropagator for
OTel. This means that we're not propagating remote-fields
(only baggage via the 'baggage' field).

With this change we're adding the missing propagator as
THE LAST entry in the composite TextMapPropagator. It has to be
last cause with the latest Snapshots of Micrometer Tracing it
will append the remote field baggage to existing baggage in the
context extracted via the W3CBaggagePropagator.

See gh-32898

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/OpenTelemetryAutoConfiguration.java

@@ -195,9 +195,11 @@ public class OpenTelemetryAutoConfiguration {
 		@Bean
 		@ConditionalOnProperty(prefix = "management.tracing.propagation", name = "type", havingValue = "W3C",
 				matchIfMissing = true)
-		TextMapPropagator w3cTextMapPropagatorWithBaggage() {
+		TextMapPropagator w3cTextMapPropagatorWithBaggage(OtelCurrentTraceContext otelCurrentTraceContext) {
+			List<String> remoteFields = this.tracingProperties.getBaggage().getRemoteFields();
 			return TextMapPropagator.composite(W3CTraceContextPropagator.getInstance(),
-					W3CBaggagePropagator.getInstance());
+					W3CBaggagePropagator.getInstance(), new BaggageTextMapPropagator(remoteFields,
+							new OtelBaggageManager(otelCurrentTraceContext, remoteFields, Collections.emptyList())));
 		}
 
 		@Bean

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/tracing/OpenTelemetryAutoConfigurationTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.autoconfigure.tracing;
 
+import java.util.Collection;
 import java.util.List;
 
 import io.micrometer.tracing.otel.bridge.OtelCurrentTraceContext;
@@ -182,7 +183,12 @@ class OpenTelemetryAutoConfigurationTests {
 
 	@Test
 	void shouldSupplyW3CPropagationWithBaggageByDefault() {
-		this.contextRunner.run((context) -> assertThat(context).hasBean("w3cTextMapPropagatorWithBaggage"));
+		this.contextRunner.withPropertyValues("management.tracing.baggage.remote-fields=foo").run((context) -> {
+			assertThat(context).hasBean("w3cTextMapPropagatorWithBaggage");
+			Collection<String> allFields = context.getBean("w3cTextMapPropagatorWithBaggage", TextMapPropagator.class)
+					.fields();
+			assertThat(allFields).containsExactly("traceparent", "tracestate", "baggage", "foo");
+		});
 	}
 
 	@Test