Fixed the kafka sender condition. fixes gh-985

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

@@ -887,9 +887,6 @@ 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

@@ -36,7 +36,7 @@ import zipkin2.reporter.kafka11.KafkaSender;
 @ConditionalOnBean(KafkaProperties.class)
 @ConditionalOnMissingBean(Sender.class)
 @Conditional(ZipkinSenderCondition.class)
-@ConditionalOnProperty(value = "spring.zipkin.kafka.enabled", havingValue = "true")
+@ConditionalOnProperty(value = "spring.zipkin.sender.type", havingValue = "kafka")
 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

@@ -1,8 +0,0 @@
-{"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

@@ -125,7 +125,7 @@ public class ZipkinAutoConfigurationTests {
 	public void overrideKafkaTopic() throws Exception {
 		context = new AnnotationConfigApplicationContext();
 		addEnvironment(context, "spring.zipkin.kafka.topic:zipkin2",
-				"spring.zipkin.kafka.enabled:true");
+				"spring.zipkin.sender.type:kafka");
 		context.register(
 				PropertyPlaceholderAutoConfiguration.class,
 				KafkaAutoConfiguration.class,