Improve "active" metrics handling in WebClient observations

Prior to this commit, the WebClient observations would have a specific
lifecycle where the observation context is build with a
`ClientRequest.Builder` as tracing needs to add an outgoing request
header before the request is made immutable.

With this setup, the metrics observation handler processes the start
event by increasing the "http.client.requests.active" counter and
collecting tags at this point. Because then the immutable request is not
yet fully built or set on the context, the keyvalues collected by the
observation convention at that point can be incomplete.

This commit ensures that a request is always made available in the
context, even if it is updated right after the observation start. The
only difference between the two should be additional tracing headers and
a request attribute holding the current observation context.

Closes gh-31702

Author: bclozel

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

@@ -50,10 +50,25 @@ public class ClientRequestObservationContext extends RequestReplySenderContext<C
 	private ClientRequest request;
 
 
+	/**
+	 * Create a new Observation context for HTTP client observations.
+	 * @deprecated as of 6.1, in favor of {@link #ClientRequestObservationContext(ClientRequest.Builder)}
+	 */
+	@Deprecated(since = "6.1", forRemoval = true)
 	public ClientRequestObservationContext() {
 		super(ClientRequestObservationContext::setRequestHeader);
 	}
 
+	/**
+	 * Create a new Observation context for HTTP client observations.
+	 */
+	public ClientRequestObservationContext(ClientRequest.Builder request) {
+		super(ClientRequestObservationContext::setRequestHeader);
+		setCarrier(request);
+		setRequest(request.build());
+	}
+
+
 	private static void setRequestHeader(@Nullable ClientRequest.Builder request, String name, String value) {
 		if (request != null) {
 			request.headers(headers -> headers.set(name, value));

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

@@ -438,12 +438,11 @@ public <V> Flux<V> exchangeToFlux(Function<ClientResponse, ? extends Flux<V>> re
 		@SuppressWarnings("deprecation")
 		@Override
 		public Mono<ClientResponse> exchange() {
-			ClientRequestObservationContext observationContext = new ClientRequestObservationContext();
 			ClientRequest.Builder requestBuilder = initRequestBuilder();
+			ClientRequestObservationContext observationContext = new ClientRequestObservationContext(requestBuilder);
 			return Mono.deferContextual(contextView -> {
 				Observation observation = ClientHttpObservationDocumentation.HTTP_REACTIVE_CLIENT_EXCHANGES.observation(observationConvention,
 						DEFAULT_OBSERVATION_CONVENTION, () -> observationContext, observationRegistry);
-				observationContext.setCarrier(requestBuilder);
 				observation
 						.parentObservation(contextView.getOrDefault(ObservationThreadLocalAccessor.KEY, null))
 						.start();
@@ -452,7 +451,7 @@ public Mono<ClientResponse> exchange() {
 					filterFunction = filterFunctions.andThen(filterFunction);
 				}
 				ClientRequest request = requestBuilder
-						.attribute(ClientRequestObservationContext.CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE, observation.getContext())
+						.attribute(ClientRequestObservationContext.CURRENT_OBSERVATION_CONTEXT_ATTRIBUTE, observationContext)
 						.build();
 				observationContext.setUriTemplate((String) request.attribute(URI_TEMPLATE_ATTRIBUTE).orElse(null));
 				observationContext.setRequest(request);

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

@@ -43,19 +43,19 @@ void shouldHaveName() {
 
 	@Test
 	void shouldHaveContextualName() {
-		ClientRequestObservationContext context = new ClientRequestObservationContext();
-		context.setCarrier(ClientRequest.create(HttpMethod.GET, URI.create("/test")));
-		context.setRequest(context.getCarrier().build());
+		ClientRequestObservationContext context = new ClientRequestObservationContext(ClientRequest.create(HttpMethod.GET, URI.create("/test")));
 		assertThat(this.observationConvention.getContextualName(context)).isEqualTo("http get");
 	}
 
 	@Test
 	void shouldOnlySupportWebClientObservationContext() {
-		assertThat(this.observationConvention.supportsContext(new ClientRequestObservationContext())).isTrue();
+		ClientRequest.Builder request = ClientRequest.create(HttpMethod.GET, URI.create("/test"));
+		assertThat(this.observationConvention.supportsContext(new ClientRequestObservationContext(request))).isTrue();
 		assertThat(this.observationConvention.supportsContext(new Observation.Context())).isFalse();
 	}
 
 	@Test
+	@SuppressWarnings("removal")
 	void shouldAddKeyValuesForNullExchange() {
 		ClientRequestObservationContext context = new ClientRequestObservationContext();
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).hasSize(6)
@@ -68,13 +68,14 @@ void shouldAddKeyValuesForNullExchange() {
 
 	@Test
 	void shouldAddKeyValuesForExchangeWithException() {
-		ClientRequestObservationContext context = new ClientRequestObservationContext();
+		ClientRequest.Builder request = ClientRequest.create(HttpMethod.GET, URI.create("/test"));
+		ClientRequestObservationContext context = new ClientRequestObservationContext(request);
 		context.setError(new IllegalStateException("Could not create client request"));
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).hasSize(6)
-				.contains(KeyValue.of("method", "none"), KeyValue.of("uri", "none"), KeyValue.of("status", "CLIENT_ERROR"),
+				.contains(KeyValue.of("method", "GET"), KeyValue.of("uri", "none"), KeyValue.of("status", "CLIENT_ERROR"),
 						KeyValue.of("client.name", "none"), KeyValue.of("exception", "IllegalStateException"), KeyValue.of("outcome", "UNKNOWN"));
 		assertThat(this.observationConvention.getHighCardinalityKeyValues(context)).hasSize(1)
-				.contains(KeyValue.of("http.url", "none"));
+				.contains(KeyValue.of("http.url", "/test"));
 	}
 
 	@Test
@@ -83,7 +84,6 @@ void shouldAddKeyValuesForRequestWithUriTemplate() {
 				.attribute(WebClient.class.getName() + ".uriTemplate", "/resource/{id}");
 		ClientRequestObservationContext context = createContext(request);
 		context.setUriTemplate("/resource/{id}");
-		context.setRequest(context.getCarrier().build());
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context))
 				.contains(KeyValue.of("exception", "none"), KeyValue.of("method", "GET"), KeyValue.of("uri", "/resource/{id}"),
 						KeyValue.of("status", "200"), KeyValue.of("client.name", "none"), KeyValue.of("outcome", "SUCCESS"));
@@ -94,7 +94,6 @@ void shouldAddKeyValuesForRequestWithUriTemplate() {
 	@Test
 	void shouldAddKeyValuesForRequestWithoutUriTemplate() {
 		ClientRequestObservationContext context = createContext(ClientRequest.create(HttpMethod.GET, URI.create("/resource/42")));
-		context.setRequest(context.getCarrier().build());
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context))
 				.contains(KeyValue.of("method", "GET"), KeyValue.of("uri", "none"));
 		assertThat(this.observationConvention.getHighCardinalityKeyValues(context)).hasSize(1).contains(KeyValue.of("http.url", "/resource/42"));
@@ -103,28 +102,24 @@ void shouldAddKeyValuesForRequestWithoutUriTemplate() {
 	@Test
 	void shouldAddClientNameKeyValueForRequestWithHost() {
 		ClientRequestObservationContext context = createContext(ClientRequest.create(HttpMethod.GET, URI.create("https://localhost:8080/resource/42")));
-		context.setRequest(context.getCarrier().build());
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).contains(KeyValue.of("client.name", "localhost"));
 	}
 
 	@Test
 	void shouldAddRootUriEvenIfTemplateMissing() {
 		ClientRequestObservationContext context = createContext(ClientRequest.create(HttpMethod.GET, URI.create("https://example.org/")));
-		context.setRequest(context.getCarrier().build());
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).contains(KeyValue.of("uri", "/"));
 	}
 
 	@Test
 	void shouldOnlyConsiderPathForUriKeyValue() {
 		ClientRequestObservationContext context = createContext(ClientRequest.create(HttpMethod.GET, URI.create("https://example.org/resource/42")));
 		context.setUriTemplate("https://example.org/resource/{id}");
-		context.setRequest(context.getCarrier().build());
 		assertThat(this.observationConvention.getLowCardinalityKeyValues(context)).contains(KeyValue.of("uri", "/resource/{id}"));
 	}
 
 	private ClientRequestObservationContext createContext(ClientRequest.Builder request) {
-		ClientRequestObservationContext context = new ClientRequestObservationContext();
-		context.setCarrier(request);
+		ClientRequestObservationContext context = new ClientRequestObservationContext(request);
 		context.setResponse(ClientResponse.create(HttpStatus.OK).build());
 		return context;
 	}