Fix bug where ZipkinRestTemplateBuilderCustomizer had no effect

Closes gh-33399

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinConfigurations.java

@@ -80,10 +80,21 @@ ZipkinRestTemplateSender restTemplateSender(ZipkinProperties properties,
 				ObjectProvider<ZipkinRestTemplateBuilderCustomizer> customizers) {
 			RestTemplateBuilder restTemplateBuilder = new RestTemplateBuilder()
 					.setConnectTimeout(properties.getConnectTimeout()).setReadTimeout(properties.getReadTimeout());
-			customizers.orderedStream().forEach((customizer) -> customizer.customize(restTemplateBuilder));
+			restTemplateBuilder = applyCustomizers(restTemplateBuilder, customizers);
 			return new ZipkinRestTemplateSender(properties.getEndpoint(), restTemplateBuilder.build());
 		}
 
+		private RestTemplateBuilder applyCustomizers(RestTemplateBuilder restTemplateBuilder,
+				ObjectProvider<ZipkinRestTemplateBuilderCustomizer> customizers) {
+			Iterable<ZipkinRestTemplateBuilderCustomizer> orderedCustomizers = () -> customizers.orderedStream()
+					.iterator();
+			RestTemplateBuilder currentBuilder = restTemplateBuilder;
+			for (ZipkinRestTemplateBuilderCustomizer customizer : orderedCustomizers) {
+				currentBuilder = customizer.customize(currentBuilder);
+			}
+			return currentBuilder;
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinRestTemplateBuilderCustomizer.java

@@ -31,7 +31,8 @@ public interface ZipkinRestTemplateBuilderCustomizer {
 	/**
 	 * Customize the rest template builder.
 	 * @param restTemplateBuilder the {@code RestTemplateBuilder} to customize
+	 * @return the customized {@code RestTemplateBuilder}
 	 */
-	void customize(RestTemplateBuilder restTemplateBuilder);
+	RestTemplateBuilder customize(RestTemplateBuilder restTemplateBuilder);
 
 }

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/tracing/zipkin/ZipkinConfigurationsSenderConfigurationTests.java

@@ -27,6 +27,7 @@
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
 import org.springframework.boot.test.context.runner.ReactiveWebApplicationContextRunner;
 import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
+import org.springframework.boot.web.client.RestTemplateBuilder;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.web.reactive.function.client.WebClient;
@@ -143,8 +144,8 @@ void shouldBackOffOnCustomBeans() {
 	private static class RestTemplateConfiguration {
 
 		@Bean
-		ZipkinRestTemplateBuilderCustomizer restTemplateBuilder() {
-			return mock(ZipkinRestTemplateBuilderCustomizer.class);
+		ZipkinRestTemplateBuilderCustomizer zipkinRestTemplateBuilderCustomizer() {
+			return new DummyZipkinRestTemplateBuilderCustomizer();
 		}
 
 	}
@@ -169,4 +170,13 @@ Sender customSender() {
 
 	}
 
+	private static class DummyZipkinRestTemplateBuilderCustomizer implements ZipkinRestTemplateBuilderCustomizer {
+
+		@Override
+		public RestTemplateBuilder customize(RestTemplateBuilder restTemplateBuilder) {
+			return restTemplateBuilder;
+		}
+
+	}
+
 }