Fix invalid Zipkin Reporter back-off behavior

Update `ReporterConfiguration` so that `spanReporter` back off
if a `Reporter` bean is defined. Prior to this commit, only
`AsyncReporter` beans would be considered.

See gh-35455

Author: be-hase

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

@@ -120,7 +120,7 @@ static class ReporterConfiguration {
 		@Bean
 		@ConditionalOnMissingBean
 		@ConditionalOnBean(Sender.class)
-		AsyncReporter<Span> spanReporter(Sender sender, BytesEncoder<Span> encoder) {
+		Reporter<Span> spanReporter(Sender sender, BytesEncoder<Span> encoder) {
 			return AsyncReporter.builder(sender).build(encoder);
 		}
 

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

@@ -56,10 +56,11 @@ void shouldNotSupplyReporterIfSenderIsMissing() {
 
 	@Test
 	void shouldBackOffOnCustomBeans() {
-		this.contextRunner.withUserConfiguration(CustomConfiguration.class).run((context) -> {
-			assertThat(context).hasBean("customReporter");
-			assertThat(context).hasSingleBean(Reporter.class);
-		});
+		this.contextRunner.withUserConfiguration(SenderConfiguration.class, CustomConfiguration.class)
+			.run((context) -> {
+				assertThat(context).hasBean("customReporter");
+				assertThat(context).hasSingleBean(Reporter.class);
+			});
 	}
 
 	@Configuration(proxyBeanMethods = false)