Propagate context in reactive HTTP server and client

Prior to this commit, the ServerHttpObservationFilter would not add
the current observation as a key in the Reactor context, preventing
from being used or propagated during the HTTP exchange handling.

Also, the client instrumentation in `DefaultWebClient` would start
the observation once the request is fully formed and immutable,
preventing the context from being propagated through HTTP request
headers.

This commit fixes both uses cases now by:

* adding the current observation as a key in the reactor context
  on the server side
* using the `ClientRequest.Builder` as a Carrier on the client side

Closes gh-29388

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

@@ -27,21 +27,24 @@ import org.springframework.lang.Nullable;
  * @author Brian Clozel
  * @since 6.0
  */
-public class ClientRequestObservationContext extends RequestReplySenderContext<ClientRequest, ClientResponse> {
+public class ClientRequestObservationContext extends RequestReplySenderContext<ClientRequest.Builder, ClientResponse> {
 
 	@Nullable
 	private String uriTemplate;
 
 	private boolean aborted;
 
+	@Nullable
+	private ClientRequest builtRequest;
+
 
 	public ClientRequestObservationContext() {
 		super(ClientRequestObservationContext::setRequestHeader);
 	}
 
-	private static void setRequestHeader(@Nullable ClientRequest request, String name, String value) {
+	private static void setRequestHeader(@Nullable ClientRequest.Builder request, String name, String value) {
 		if (request != null) {
-			request.headers().set(name, value);
+			request.header(name, value);
 		}
 	}
 
@@ -75,4 +78,18 @@ public class ClientRequestObservationContext extends RequestReplySenderContext<C
 	void setAborted(boolean aborted) {
 		this.aborted = aborted;
 	}
+
+	/**
+	 * Return the built request.
+	 */
+	public ClientRequest getBuiltRequest() {
+		return this.builtRequest;
+	}
+
+	/**
+	 * Set the built request.
+	 */
+	public void setBuiltRequest(ClientRequest builtRequest) {
+		this.builtRequest = builtRequest;
+	}
 }

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

@@ -79,7 +79,7 @@ public class DefaultClientRequestObservationConvention implements ClientRequestO
 
 	@Override
 	public String getContextualName(ClientRequestObservationContext context) {
-		return "http " + context.getCarrier().method().name().toLowerCase();
+		return "http " + context.getBuiltRequest().method().name().toLowerCase();
 	}
 
 	@Override
@@ -95,8 +95,8 @@ public class DefaultClientRequestObservationConvention implements ClientRequestO
 	}
 
 	protected KeyValue method(ClientRequestObservationContext context) {
-		if (context.getCarrier() != null) {
-			return KeyValue.of(ClientHttpObservationDocumentation.LowCardinalityKeyNames.METHOD, context.getCarrier().method().name());
+		if (context.getBuiltRequest() != null) {
+			return KeyValue.of(ClientHttpObservationDocumentation.LowCardinalityKeyNames.METHOD, context.getBuiltRequest().method().name());
 		}
 		else {
 			return METHOD_NONE;
@@ -143,15 +143,15 @@ public class DefaultClientRequestObservationConvention implements ClientRequestO
 	}
 
 	protected KeyValue httpUrl(ClientRequestObservationContext context) {
-		if (context.getCarrier() != null) {
-			return KeyValue.of(ClientHttpObservationDocumentation.HighCardinalityKeyNames.HTTP_URL, context.getCarrier().url().toASCIIString());
+		if (context.getBuiltRequest() != null) {
+			return KeyValue.of(ClientHttpObservationDocumentation.HighCardinalityKeyNames.HTTP_URL, context.getBuiltRequest().url().toASCIIString());
 		}
 		return HTTP_URL_NONE;
 	}
 
 	protected KeyValue clientName(ClientRequestObservationContext context) {
-		if (context.getCarrier() != null && context.getCarrier().url().getHost() != null) {
-			return KeyValue.of(ClientHttpObservationDocumentation.HighCardinalityKeyNames.CLIENT_NAME, context.getCarrier().url().getHost());
+		if (context.getBuiltRequest() != null && context.getBuiltRequest().url().getHost() != null) {
+			return KeyValue.of(ClientHttpObservationDocumentation.HighCardinalityKeyNames.CLIENT_NAME, context.getBuiltRequest().url().getHost());
 		}
 		return CLIENT_NAME_NONE;
 	}

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

@@ -75,6 +75,11 @@ class DefaultWebClient implements WebClient {
 
 	private static final DefaultClientRequestObservationConvention DEFAULT_OBSERVATION_CONVENTION = new DefaultClientRequestObservationConvention();
 
+	/**
+	 * Aligned with ObservationThreadLocalAccessor#KEY from micrometer-core.
+	 */
+	private static final String MICROMETER_OBSERVATION = "micrometer.observation";
+
 	private final ExchangeFunction exchangeFunction;
 
 	private final UriBuilderFactory uriBuilderFactory;
@@ -450,14 +455,19 @@ class DefaultWebClient implements WebClient {
 		@SuppressWarnings("deprecation")
 		public Mono<ClientResponse> exchange() {
 			ClientRequestObservationContext observationContext = new ClientRequestObservationContext();
-			ClientRequest request = (this.inserter != null ?
-					initRequestBuilder().body(this.inserter).build() :
-					initRequestBuilder().build());
-			return Mono.defer(() -> {
+			ClientRequest.Builder requestBuilder = this.inserter != null ?
+					initRequestBuilder().body(this.inserter) :
+					initRequestBuilder();
+			return Mono.deferContextual(contextView -> {
 				Observation observation = ClientHttpObservationDocumentation.HTTP_REQUEST.observation(observationConvention,
-						DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, observationRegistry).start();
-				observationContext.setCarrier(request);
+						DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, observationRegistry);
+				observationContext.setCarrier(requestBuilder);
+				observation
+						.parentObservation(contextView.getOrDefault(MICROMETER_OBSERVATION, null))
+						.start();
+				ClientRequest request = requestBuilder.build();
 				observationContext.setUriTemplate((String) request.attribute(URI_TEMPLATE_ATTRIBUTE).orElse(null));
+				observationContext.setBuiltRequest(request);
 				Mono<ClientResponse> responseMono = exchangeFunction.exchange(request)
 						.checkpoint("Request to " + this.httpMethod.name() + " " + this.uri + " [DefaultWebClient]")
 						.switchIfEmpty(NO_HTTP_CLIENT_RESPONSE_ERROR);