Polish "Fix invalid Zipkin Reporter back-off behavior"

See gh-35455

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

@@ -118,9 +118,9 @@ class ZipkinConfigurations {
 	static class ReporterConfiguration {
 
 		@Bean
-		@ConditionalOnMissingBean
+		@ConditionalOnMissingBean(Reporter.class)
 		@ConditionalOnBean(Sender.class)
-		Reporter<Span> spanReporter(Sender sender, BytesEncoder<Span> encoder) {
+		AsyncReporter<Span> spanReporter(Sender sender, BytesEncoder<Span> encoder) {
 			return AsyncReporter.builder(sender).build(encoder);
 		}