WebClient instrumentation fails with IllegalArgumentException when adapting to WebClientExchangeTagsProvider

[bdeneuter]

We are upgrading to Spring Boot 3.0 and Spring framework 6.0.2. We have observability activated in our services and are using DataDog and Prometheus.
When using the WebClient, we receive the following Exception:

java.lang.IllegalArgumentException: ClientRequest must not be null
	at org.springframework.util.Assert.notNull(Assert.java:204)
	Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below: 
Original Stack Trace:
		at app//org.springframework.util.Assert.notNull(Assert.java:204)
		at app//org.springframework.web.reactive.function.client.DefaultClientRequestBuilder.<init>(DefaultClientRequestBuilder.java:72)
		at app//org.springframework.web.reactive.function.client.ClientRequest.from(ClientRequest.java:134)
		at app//org.springframework.boot.actuate.autoconfigure.observation.web.client.ClientObservationConventionAdapter.mutateClientRequest(ClientObservationConventionAdapter.java:66)
		at app//org.springframework.boot.actuate.autoconfigure.observation.web.client.ClientObservationConventionAdapter.getLowCardinalityKeyValues(ClientObservationConventionAdapter.java:58)
		at app//org.springframework.boot.actuate.autoconfigure.observation.web.client.ClientObservationConventionAdapter.getLowCardinalityKeyValues(ClientObservationConventionAdapter.java:36)
		at app//io.micrometer.observation.SimpleObservation.start(SimpleObservation.java:134)
		at app//org.springframework.web.reactive.function.client.DefaultWebClient$DefaultRequestBodyUriSpec.lambda$exchange$9(DefaultWebClient.java:465)
		at app//reactor.core.publisher.MonoDeferContextual.subscribe(MonoDeferContextual.java:47)
		at app//reactor.core.publisher.InternalMonoOperator.subscribe(InternalMonoOperator.java:64)
		at app//reactor.core.publisher.FluxRetryWhen.subscribe(FluxRetryWhen.java:77)
		at app//reactor.core.publisher.MonoRetryWhen.subscribeOrReturn(MonoRetryWhen.java:46)
		at app//reactor.core.publisher.Mono.subscribe(Mono.java:4429)
		at app//reactor.core.publisher.Mono.block(Mono.java:1709)

It seems that WebClient starts an Observation:
https://github.com/spring-projects/spring-framework/blob/main/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java#L465

observation
     .parentObservation(contextView.getOrDefault(ObservationThreadLocalAccessor.KEY, null))
     .start();

This observation uses the ClientRequest in:

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

Line 66 in 0acaa0e

ClientRequest clientRequest = ClientRequest.from(context.getRequest())

ClientRequest clientRequest = ClientRequest.from(context.getRequest())
				.attribute(URI_TEMPLATE_ATTRIBUTE, context.getUriTemplate()).build();

But it is set afterwards in:
https://github.com/spring-projects/spring-framework/blob/main/spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java#L468

ClientRequest request = requestBuilder.build();
				observationContext.setUriTemplate((String) request.attribute(URI_TEMPLATE_ATTRIBUTE).orElse(null));
				observationContext.setRequest(request);

Causing this IllegalArgumentException.

[bdeneuter]

It seems to be triggered by our custom tags that we set on the WebClient with our own Spring bean that extends from org.springframework.boot.actuate.metrics.web.servlet.DefaultWebMvcTagsProvider

[bclozel]

Thanks @bdeneuter for your report, this fix will be released in the next maintenance version.