Complete Propagator.Getter implementation

As of Micrometer Tracing 1.6.0, the `Propagator.Getter` interface
adds a new `getAll` method with a default implementation return a
singleton collection.

This commit adds the missing implementation override in both Servlet and
Reactor web server contexts.

Fixes gh-35965

spring-web/src/main/java/org/springframework/http/server/observation/ServerRequestObservationContext.java

@@ -16,6 +16,9 @@
 
 package org.springframework.http.server.observation;
 
+import java.util.Collections;
+
+import io.micrometer.observation.transport.Propagator;
 import io.micrometer.observation.transport.RequestReplyReceiverContext;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.HttpServletResponse;
@@ -32,10 +35,12 @@ import org.jspecify.annotations.Nullable;
  */
 public class ServerRequestObservationContext extends RequestReplyReceiverContext<HttpServletRequest, HttpServletResponse> {
 
+	private static final HeaderGetter GETTER = new HeaderGetter();
+
 	private @Nullable String pathPattern;
 
 	public ServerRequestObservationContext(HttpServletRequest request, HttpServletResponse response) {
-		super(HttpServletRequest::getHeader);
+		super(GETTER);
 		setCarrier(request);
 		setResponse(response);
 	}
@@ -47,4 +52,17 @@ public class ServerRequestObservationContext extends RequestReplyReceiverContext
 	public void setPathPattern(@Nullable String pathPattern) {
 		this.pathPattern = pathPattern;
 	}
+
+	static final class HeaderGetter implements Propagator.Getter<HttpServletRequest> {
+		@Override
+		public String get(HttpServletRequest carrier, String key) {
+			return carrier.getHeader(key);
+		}
+
+		@Override
+		public Iterable<String> getAll(HttpServletRequest carrier, String key) {
+			return Collections.list(carrier.getHeaders(key));
+		}
+	}
+
 }

spring-web/src/main/java/org/springframework/http/server/reactive/observation/ServerRequestObservationContext.java

@@ -20,6 +20,7 @@ import java.util.Collections;
 import java.util.Map;
 import java.util.Optional;
 
+import io.micrometer.observation.transport.Propagator;
 import io.micrometer.observation.transport.RequestReplyReceiverContext;
 import org.jspecify.annotations.Nullable;
 
@@ -46,6 +47,7 @@ public class ServerRequestObservationContext extends RequestReplyReceiverContext
 	 */
 	public static final String CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE = ServerRequestObservationContext.class.getName();
 
+	private static final HeaderGetter GETTER = new HeaderGetter();
 
 	private final Map<String, Object> attributes;
 
@@ -63,7 +65,7 @@ public class ServerRequestObservationContext extends RequestReplyReceiverContext
 	public ServerRequestObservationContext(
 			ServerHttpRequest request, ServerHttpResponse response, Map<String, Object> attributes) {
 
-		super((req, key) -> req.getHeaders().getFirst(key));
+		super(GETTER);
 		setCarrier(request);
 		setResponse(response);
 		this.attributes = Collections.unmodifiableMap(attributes);
@@ -129,4 +131,17 @@ public class ServerRequestObservationContext extends RequestReplyReceiverContext
 				(ServerRequestObservationContext) attributes.get(CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE));
 	}
 
+	static final class HeaderGetter implements Propagator.Getter<ServerHttpRequest> {
+
+		@Override
+		public @Nullable String get(ServerHttpRequest carrier, String key) {
+			return carrier.getHeaders().getFirst(key);
+		}
+
+		@Override
+		public Iterable<String> getAll(ServerHttpRequest carrier, String key) {
+			return carrier.getHeaders().getOrEmpty(key);
+		}
+	}
+
 }

spring-web/src/test/java/org/springframework/http/server/observation/ServerRequestObservationContextTests.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2025-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.http.server.observation;
+
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
+import org.springframework.web.testfixture.servlet.MockHttpServletResponse;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ServerRequestObservationContext}.
+ */
+class ServerRequestObservationContextTests {
+
+	private MockHttpServletRequest request = new MockHttpServletRequest();
+
+	private MockHttpServletResponse response = new MockHttpServletResponse();
+
+	private ServerRequestObservationContext context = new ServerRequestObservationContext(request, response);
+
+	@Test
+	void shouldGetMultipleHeaderValues() {
+		request.addHeader("test", List.of("spring", "framework"));
+		assertThat(context.getGetter().getAll(request, "test")).contains("spring", "framework");
+	}
+
+}

spring-web/src/test/java/org/springframework/http/server/reactive/observation/ServerRequestObservationContextTests.java

@@ -0,0 +1,44 @@
+/*
+ * Copyright 2025-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.http.server.reactive.observation;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest;
+import org.springframework.web.testfixture.server.MockServerWebExchange;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ServerRequestObservationContext}.
+ */
+class ServerRequestObservationContextTests {
+
+
+	@Test
+	void shouldGetMultipleHeaderValues() {
+		MockServerHttpRequest request = MockServerHttpRequest.get("/")
+				.header("test", "spring", "framework").build();
+		MockServerWebExchange exchange = MockServerWebExchange.builder(request).build();
+
+		ServerRequestObservationContext context = new ServerRequestObservationContext(request, exchange.getResponse(), Map.of());
+		assertThat(context.getGetter().getAll(request, "test")).contains("spring", "framework");
+	}
+
+}