ZipkinRestTemplateSender is not customizable

[natrem]

In Spring Boot 3.0.0, ZipkinConfigurations.RestTemplateSenderConfiguration create a RestTemplate by relying on RestTemplateBuilder.
The contract indicates that providing a ZipkinRestTemplateBuilderCustomizer would allow for customization of the generated RestTemplate.

However, RestTemplateBuilder is immutable and each method returns a new instance (contrary to the WebClient version).
As a consequence, customization is impossible.

Example failing test:

	@Test
	void should_apply_interceptors() {
		ClientHttpRequestInterceptor interceptor1 = mock(ClientHttpRequestInterceptor.class);
		ZipkinRestTemplateBuilderCustomizer customizer = builder -> builder.additionalInterceptors(interceptor1);
		RestTemplate restTemplate = restTemplateSender(new ZipkinProperties(), new TestObjectProvider<>(customizer));

		// empty list
		assertThat(restTemplate.getInterceptors()).singleElement().isEqualTo(interceptor1);
	}

	// copied from org.springframework.boot.actuate.autoconfigure.tracing.zipkin.ZipkinConfigurations.RestTemplateSenderConfiguration#restTemplateSender
	RestTemplate restTemplateSender(ZipkinProperties properties,
		ObjectProvider<ZipkinRestTemplateBuilderCustomizer> customizers) {
		RestTemplateBuilder restTemplateBuilder = new RestTemplateBuilder()
			.setConnectTimeout(properties.getConnectTimeout()).setReadTimeout(properties.getReadTimeout());

		customizers.orderedStream().forEach((customizer) -> customizer.customize(restTemplateBuilder));

		return restTemplateBuilder.build();
	}

	@RequiredArgsConstructor
	static class TestObjectProvider<T> implements ObjectProvider<T> {
		private final T t;

		@Override
		public T getObject(Object... args) throws BeansException {
			return t;
		}

		@Override
		public T getIfAvailable() throws BeansException {
			return t;
		}

		@Override
		public T getIfUnique() throws BeansException {
			return t;
		}

		@Override
		public T getObject() throws BeansException {
			return t;
		}

		@Override
		public Stream<T> orderedStream() {
			return Stream.of(t);
		}
	}

I think that it is required to make ZipkinRestTemplateBuilderCustomizer return a RestTemplateBuilder to have a working solution, or mutate the RestTemplate instead of the RestTemplateBuilder.

Work-around:

1. Define your own ZipkinRestTemplateSender (which requires duplicating a lot of classes since ZipkinRestTemplateSender is package private)
2. Use the web client version, which should not have this problem.

[wilkinsona]

I think we should change the signature of the customize method from void customize(RestTemplateBuilder restTemplateBuilder) to RestTemplateBuilder customize(RestTemplateBuilder restTemplateBuilder). While it is technically a breaking change, I think it's OK as the method's useless in its current form.

[wilkinsona]

We discussed this today and decided to change customize to return RestTemplateBuilder .