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/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");
+	}
+
+}