Update RestClientBeanPostProcessor/WebClientBeanPostProcessor to create new bean only when adding OTEL Interceptor

[lenin-jaganathan]

The RestClientBeanPostProcessor was always creating a new RestClient bean via mutate().build(), even when the OpenTelemetry interceptor was already present. This resulted in unnecessary bean creation.

Fixes #15545

[linux-foundation-easycla]

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• ✅ login: lenin-jaganathan / name: Lenin Jaganathan (79ec6d8)

[lenin-jaganathan]

This is the cleaner approach to add instrumentation and is also consistent with what is being done in RestClient.

But doing so would mean we have to add different modules for spring4 where the customizer is moved to the spring-boot-webclient module. What are the thoughts here?

[github-actions]

This PR has been labeled as stale due to lack of activity and needing author feedback. It will be automatically closed if there is no further activity over the next 7 days.

[github-actions]

This PR has been labeled as stale due to lack of activity and needing author feedback. It will be automatically closed if there is no further activity over the next 7 days.

[jaydeluca]

@lenin-jaganathan in case you might not have noticed, there are some merge conflicts that need to be resolved

[lenin-jaganathan]

@lenin-jaganathan in case you might not have noticed, there are some merge conflicts that need to be resolved

Just now coming off a break and saw it. I have fixed it and pushed changes.

[laurit]

could parameterize this class

  AbstractWebClientCustomizerAutoConfigurationTest<T>
  protected abstract Class<T> webClientCustomizerClass();
  protected abstract void customizeWebClient(T customizer, WebClient.Builder builder);

subclasses could then use

extends AbstractWebClientCustomizerAutoConfigurationTest<WebClientCustomizer>
protected Class<WebClientCustomizer> webClientCustomizerClass()
protected void customizeWebClient(WebClientCustomizer customizer, WebClient.Builder builder)

[lenin-jaganathan]

Addressed

[jaydeluca]

nitpick: expectedCount isn't necessary since it's always 1

[laurit]

Do I understand correctly that the intention here is that the interceptor will be added here when the WebClient wasn't created by spring. For example when there is something like

@Bean
WebClient webClient() {
  return WebClient.create();
}

I'd guess the WebClientCustomizer won't run then. What if there is

@Bean
WebClient.Builder webClientBuilder() {
  return WebClient.builder();
}

I'd guess the WebClientCustomizer won't run then either. Should we handle that? Should this be aligned with RestClientBeanPostProcessor (cc @zeitlinger, do you remember why you wrote RestClientBeanPostProcessor the way it is).
Assuming that the intent is to add the interceptor in post processor when the customizer didn't run do we have any tests for this?