Kafka sender needs to be explicitly set

default sender is Kafka when it is detected on the classpath, with this change it will be Web. You have to set `spring.zipkin.kafka.enabled` to `true` to use Kafka

fixes gh-985

docs/src/main/asciidoc/spring-cloud-sleuth.adoc

@@ -887,6 +887,9 @@ The following example shows setting the sender type for `web`:
 spring.zipkin.sender.type: web
 ----
 
+IMPORTANT: If you have Kafka on the classpath, you need to set
+`spring.zipkin.kafka.enabled` to `true` apart from setting the sender type.
+
 To customize the `RestTemplate` that sends spans to Zipkin via HTTP, you can register
 the `ZipkinRestTemplateCustomizer` bean.
 

spring-cloud-sleuth-zipkin/src/main/java/org/springframework/cloud/sleuth/zipkin2/sender/ZipkinKafkaSenderConfiguration.java

@@ -23,6 +23,7 @@ import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.autoconfigure.kafka.KafkaProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Conditional;
@@ -35,6 +36,7 @@ import zipkin2.reporter.kafka11.KafkaSender;
 @ConditionalOnBean(KafkaProperties.class)
 @ConditionalOnMissingBean(Sender.class)
 @Conditional(ZipkinSenderCondition.class)
+@ConditionalOnProperty(value = "spring.zipkin.kafka.enabled", havingValue = "true")
 class ZipkinKafkaSenderConfiguration {
 	@Value("${spring.zipkin.kafka.topic:zipkin}")
 	private String topic;

spring-cloud-sleuth-zipkin/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -0,0 +1,8 @@
+{"properties": [
+  {
+    "name": "spring.zipkin.kafka.enabled",
+    "type": "java.lang.Boolean",
+    "description": "Should Kafka Zipkin sender be enabled. With Kafka on the classpath you have to set this value to true to send spans to Zipkin via Kafka.",
+    "defaultValue": false
+  }
+]}

spring-cloud-sleuth-zipkin/src/test/java/org/springframework/cloud/sleuth/zipkin2/ZipkinAutoConfigurationTests.java

@@ -124,7 +124,8 @@ public class ZipkinAutoConfigurationTests {
 	@Test
 	public void overrideKafkaTopic() throws Exception {
 		context = new AnnotationConfigApplicationContext();
-		addEnvironment(context, "spring.zipkin.kafka.topic:zipkin2");
+		addEnvironment(context, "spring.zipkin.kafka.topic:zipkin2",
+				"spring.zipkin.kafka.enabled:true");
 		context.register(
 				PropertyPlaceholderAutoConfiguration.class,
 				KafkaAutoConfiguration.class,