Guard against no-op observation

Update `ServerHttpObservationFilter` to check if the `Observation`
is a no-op before adding the `ServerRequestObservationContext`.

Prior to this commit, if the `Observation` is a no-op then the
context type added with the `CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE`
would not be a `ServerRequestObservationContext`. This would mean
that `findObservationContext` would throw a `ClassCastException`.

Fixes gh-29356

Author: philwebb

spring-web/src/main/java/org/springframework/web/filter/ServerHttpObservationFilter.java

@@ -130,7 +130,9 @@ private Observation createOrFetchObservation(HttpServletRequest request, HttpSer
 			observation = ServerHttpObservationDocumentation.HTTP_REQUESTS.observation(this.observationConvention,
 					DEFAULT_OBSERVATION_CONVENTION, () -> context, this.observationRegistry).start();
 			request.setAttribute(CURRENT_OBSERVATION_ATTRIBUTE, observation);
-			request.setAttribute(CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE, observation.getContext());
+			if (!observation.isNoop()) {
+				request.setAttribute(CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE, observation.getContext());
+			}
 		}
 		return observation;
 	}

spring-web/src/test/java/org/springframework/web/filter/ServerHttpObservationFilterTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.web.filter;
 
+import io.micrometer.observation.ObservationRegistry;
 import io.micrometer.observation.tck.TestObservationRegistry;
 import io.micrometer.observation.tck.TestObservationRegistryAssert;
 import jakarta.servlet.RequestDispatcher;
@@ -33,6 +34,7 @@
 
 /**
  * Tests for {@link ServerHttpObservationFilter}.
+ *
  * @author Brian Clozel
  */
 class ServerHttpObservationFilterTests {
@@ -61,6 +63,16 @@ void filterShouldFillObservationContext() throws Exception {
 		assertThatHttpObservation().hasLowCardinalityKeyValue("outcome", "SUCCESS");
 	}
 
+	@Test
+	void filterShouldAcceptNoOpObservationContext() throws Exception {
+		ServerHttpObservationFilter filter = new ServerHttpObservationFilter(ObservationRegistry.NOOP);
+		filter.doFilter(this.request, this.response, this.mockFilterChain);
+
+		ServerRequestObservationContext context = (ServerRequestObservationContext) this.request
+				.getAttribute(ServerHttpObservationFilter.CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE);
+		assertThat(context).isNull();
+	}
+
 	@Test
 	void filterShouldUseThrownException() throws Exception {
 		IllegalArgumentException customError = new IllegalArgumentException("custom error");