Fixes lack of load balanced URL extraction for Zipkin and Reactor

fixes gh-2339

spring-cloud-sleuth-autoconfigure/src/main/java/org/springframework/cloud/sleuth/autoconfig/zipkin2/ZipkinHttpSenderConfiguration.java

@@ -16,6 +16,8 @@
 
 package org.springframework.cloud.sleuth.autoconfig.zipkin2;
 
+import java.net.URI;
+
 import zipkin2.reporter.Sender;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -82,9 +84,11 @@ class ZipkinHttpSenderConfiguration {
 	static class ZipkinReactiveConfiguration {
 
 		@Bean(ZipkinAutoConfiguration.SENDER_BEAN_NAME)
-		Sender webClientSender(ZipkinProperties zipkin, ZipkinWebClientBuilderProvider zipkinWebClientBuilderProvider) {
+		Sender webClientSender(ZipkinProperties zipkin, ZipkinWebClientBuilderProvider zipkinWebClientBuilderProvider,
+				ZipkinUrlExtractor zipkinUrlExtractor) {
 			WebClient.Builder webClientBuilder = zipkinWebClientBuilderProvider.zipkinWebClientBuilder();
-			return new WebClientSender(webClientBuilder.build(), zipkin.getBaseUrl(), zipkin.getApiPath(),
+			URI uri = zipkinUrlExtractor.zipkinUrl(zipkin);
+			return new WebClientSender(webClientBuilder.build(), uri.toString(), zipkin.getApiPath(),
 					zipkin.getEncoder(), zipkin.getCheckTimeout());
 		}
 
@@ -94,6 +98,11 @@ class ZipkinHttpSenderConfiguration {
 			return WebClient::builder;
 		}
 
+		@Bean
+		ZipkinUrlExtractor defaultZipkinUrlExtractor(final ZipkinLoadBalancer zipkinLoadBalancer) {
+			return new CachingZipkinUrlExtractor(zipkinLoadBalancer);
+		}
+
 	}
 
 	@Configuration(proxyBeanMethods = false)

spring-cloud-sleuth-autoconfigure/src/test/java/org/springframework/cloud/sleuth/autoconfig/zipkin2/ZipkinHttpSenderConfigurationReactiveTests.java

@@ -49,7 +49,7 @@ class ZipkinHttpSenderConfigurationReactiveTests {
 		springApplication.setWebApplicationType(WebApplicationType.REACTIVE);
 
 		try (ConfigurableApplicationContext context = springApplication.run("--spring.sleuth.noop.enabled=true",
-				"--server.port=0")) {
+				"--server.port=0", "--spring.zipkin.base-url=http://cloudUrl")) {
 			then(context.getBean(Sender.class)).isInstanceOf(WebClientSender.class);
 		}
 	}