Fix IllegalArgumentException in WebClient convention adapter

Prior to this commit, the `ClientObservationConventionAdapter` would
fail with an `IllegalArgumentException` when the observation is first
started: at this point, the carrier (the request builder here) is
present, but the full request not yet fully built.

This commit ensures that the convention adapter uses the request and, if
not available, the request builder to adapt to the
`WebClientExchangeTagsProvider`.

Fixes gh-33483

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/observation/web/client/ClientObservationConventionAdapter.java

@@ -55,19 +55,14 @@ public boolean supportsContext(Observation.Context context) {
 
 	@Override
 	public KeyValues getLowCardinalityKeyValues(ClientRequestObservationContext context) {
-		mutateClientRequest(context);
-		Iterable<Tag> tags = this.tagsProvider.tags(context.getRequest(), context.getResponse(), context.getError());
+		ClientRequest request = context.getRequest();
+		if (request == null) {
+			request = context.getCarrier().attribute(URI_TEMPLATE_ATTRIBUTE, context.getUriTemplate()).build();
+		}
+		Iterable<Tag> tags = this.tagsProvider.tags(request, context.getResponse(), context.getError());
 		return KeyValues.of(tags, Tag::getKey, Tag::getValue);
 	}
 
-	private void mutateClientRequest(ClientRequestObservationContext context) {
-		// WebClientExchangeTagsProvider relies on a request attribute to get the URI
-		// template, we need to adapt to that.
-		ClientRequest clientRequest = ClientRequest.from(context.getRequest())
-				.attribute(URI_TEMPLATE_ATTRIBUTE, context.getUriTemplate()).build();
-		context.setRequest(clientRequest);
-	}
-
 	@Override
 	public KeyValues getHighCardinalityKeyValues(ClientRequestObservationContext context) {
 		return KeyValues.empty();

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/observation/web/client/ClientObservationConventionAdapterTests.java

@@ -29,6 +29,7 @@
 import org.springframework.web.reactive.function.client.ClientRequest;
 import org.springframework.web.reactive.function.client.ClientRequestObservationContext;
 import org.springframework.web.reactive.function.client.ClientResponse;
+import org.springframework.web.reactive.function.client.WebClient;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -45,7 +46,8 @@ class ClientObservationConventionAdapterTests {
 	private ClientObservationConventionAdapter convention = new ClientObservationConventionAdapter(TEST_METRIC_NAME,
 			new DefaultWebClientExchangeTagsProvider());
 
-	private ClientRequest.Builder requestBuilder = ClientRequest.create(HttpMethod.GET, URI.create("/resource/test"));
+	private ClientRequest.Builder requestBuilder = ClientRequest.create(HttpMethod.GET, URI.create("/resource/test"))
+			.attribute(WebClient.class.getName() + ".uriTemplate", "/resource/{name}");
 
 	private ClientResponse response = ClientResponse.create(HttpStatus.OK).body("foo").build();
 
@@ -55,7 +57,6 @@ class ClientObservationConventionAdapterTests {
 	void setup() {
 		this.context = new ClientRequestObservationContext();
 		this.context.setCarrier(this.requestBuilder);
-		this.context.setRequest(this.requestBuilder.build());
 		this.context.setResponse(this.response);
 		this.context.setUriTemplate("/resource/{name}");
 	}
@@ -73,6 +74,14 @@ void shouldOnlySupportClientObservationContext() {
 
 	@Test
 	void shouldPushTagsAsLowCardinalityKeyValues() {
+		this.context.setRequest(this.requestBuilder.build());
+		assertThat(this.convention.getLowCardinalityKeyValues(this.context)).contains(KeyValue.of("status", "200"),
+				KeyValue.of("outcome", "SUCCESS"), KeyValue.of("uri", "/resource/{name}"),
+				KeyValue.of("method", "GET"));
+	}
+
+	@Test
+	void doesNotFailWithEmptyRequest() {
 		assertThat(this.convention.getLowCardinalityKeyValues(this.context)).contains(KeyValue.of("status", "200"),
 				KeyValue.of("outcome", "SUCCESS"), KeyValue.of("uri", "/resource/{name}"),
 				KeyValue.of("method", "GET"));