Add current observation context in ClientRequest

Prior to this commit, `ExchangeFilterFunction` could only get the
current observation from the reactor context. This is particularly
useful when such filters want to add KeyValues to the observation
context.

This commit makes this use case easier by adding the context of the
current observation as a request attribute. This also aligns the
behavior with other instrumentations.

Fixes gh-31646

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/ClientRequestObservationContext.java

@@ -16,6 +16,8 @@
 
 package org.springframework.web.reactive.function.client;
 
+import java.util.Optional;
+
 import io.micrometer.observation.transport.RequestReplySenderContext;
 
 import org.springframework.lang.Nullable;
@@ -32,6 +34,13 @@ import org.springframework.lang.Nullable;
  */
 public class ClientRequestObservationContext extends RequestReplySenderContext<ClientRequest.Builder, ClientResponse> {
 
+	/**
+	 * Name of the request attribute holding the {@link ClientRequestObservationContext context}
+	 * for the current observation.
+	 * @since 6.1.1
+	 */
+	public static final String CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE = ClientRequestObservationContext.class.getName();
+
 	@Nullable
 	private String uriTemplate;
 
@@ -96,4 +105,15 @@ public class ClientRequestObservationContext extends RequestReplySenderContext<C
 	public void setRequest(ClientRequest request) {
 		this.request = request;
 	}
+
+	/**
+	 * Get the current {@link ClientRequestObservationContext observation context}
+	 * from the given request, if available.
+	 * @param request the current client request
+	 * @return the current observation context
+	 * @since 6.1.2
+	 */
+	public static Optional<ClientRequestObservationContext> findCurrent(ClientRequest request) {
+		return Optional.ofNullable((ClientRequestObservationContext) request.attributes().get(CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE));
+	}
 }

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -450,7 +450,9 @@ final class DefaultWebClient implements WebClient {
 				if (filterFunctions != null) {
 					filterFunction = filterFunctions.andThen(filterFunction);
 				}
-				ClientRequest request = requestBuilder.build();
+				ClientRequest request = requestBuilder
+						.attribute(ClientRequestObservationContext.CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE, observation.getContext())
+						.build();
 				observationContext.setUriTemplate((String) request.attribute(URI_TEMPLATE_ATTRIBUTE).orElse(null));
 				observationContext.setRequest(request);
 				Mono<ClientResponse> responseMono = filterFunction.apply(exchangeFunction)

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientObservationTests.java

@@ -152,6 +152,26 @@ class WebClientObservationTests {
 		verifyAndGetRequest();
 	}
 
+	@Test
+	void setsCurrentObservationContextAsRequestAttribute() {
+		ExchangeFilterFunction assertionFilter = new ExchangeFilterFunction() {
+			@Override
+			public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction chain) {
+				Optional<ClientRequestObservationContext> observationContext = ClientRequestObservationContext.findCurrent(request);
+				assertThat(observationContext).isPresent();
+				return chain.exchange(request).contextWrite(context -> {
+					Observation currentObservation = context.get(ObservationThreadLocalAccessor.KEY);
+					assertThat(currentObservation.getContext()).isEqualTo(observationContext.get());
+					return context;
+				});
+			}
+		};
+		this.builder.filter(assertionFilter).build().get().uri("/resource/{id}", 42)
+				.retrieve().bodyToMono(Void.class)
+				.block(Duration.ofSeconds(10));
+		verifyAndGetRequest();
+	}
+
 	@Test
 	void recordsObservationWithResponseDetailsWhenFilterFunctionErrors() {
 		ExchangeFilterFunction errorFunction = (req, next) -> next.exchange(req).then(Mono.error(new IllegalStateException()));