Moves responsibility to import SamplerAutoConfiguration to core

Before, spring-cloud-sleuth-zipkin had to import `SamplerAutoConfiguration`
directly to unwind a sampler ordering problem caused by `TraceAutoConfiguration`
defining the default `Sampler` bean.

This fixes that by moving the default `Sampler` to where it belongs
(`SamplerAutoConfiguration`) and having `TraceAutoConfiguration` import
the sampling configuration directly as opposed to relying on auto-configuration
ordering. Finally it removes the mistake of setting `SamplerAutoConfiguration`
as auto-configuration in the first place.

The name `SamplerAutoConfiguration` was left alone because changing it would
interfere with 3rd party code that formerly imported it to correct this issue
in their non-zipkin setups.

Fixes #1618

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/autoconfig/TraceAutoConfiguration.java

@@ -49,8 +49,10 @@ import org.springframework.boot.context.properties.EnableConfigurationProperties
 import org.springframework.cloud.sleuth.DefaultSpanNamer;
 import org.springframework.cloud.sleuth.SpanAdjuster;
 import org.springframework.cloud.sleuth.SpanNamer;
+import org.springframework.cloud.sleuth.sampler.SamplerAutoConfiguration;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.context.annotation.Import;
 import org.springframework.lang.Nullable;
 import org.springframework.util.StringUtils;
 
@@ -66,6 +68,7 @@ import org.springframework.util.StringUtils;
 @Configuration
 @ConditionalOnProperty(value = "spring.sleuth.enabled", matchIfMissing = true)
 @EnableConfigurationProperties(SleuthProperties.class)
+@Import(SamplerAutoConfiguration.class)
 public class TraceAutoConfiguration {
 
 	/**
@@ -131,12 +134,6 @@ public class TraceAutoConfiguration {
 		return tracing.tracer();
 	}
 
-	@Bean
-	@ConditionalOnMissingBean
-	Sampler sleuthTraceSampler() {
-		return Sampler.NEVER_SAMPLE;
-	}
-
 	@Bean
 	@ConditionalOnMissingBean
 	SpanNamer sleuthSpanNamer() {

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/sampler/SamplerAutoConfiguration.java

@@ -20,24 +20,30 @@ import brave.sampler.Sampler;
 
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
-import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.cloud.context.config.annotation.RefreshScope;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 
 /**
- * {@link org.springframework.boot.autoconfigure.EnableAutoConfiguration
- * Auto-configuration} to setup sampling for Spring Cloud Sleuth.
+ * {@linkplain Configuration configuration} for {@link Sampler}.
  *
  * @author Marcin Grzejszczak
  * @since 2.1.0
  */
 @Configuration
-@ConditionalOnProperty(value = "spring.sleuth.enabled", matchIfMissing = true)
 @EnableConfigurationProperties(SamplerProperties.class)
+// This is not auto-configuration, but it was in the past. Leaving the name as
+// SamplerAutoConfiguration because those not using Zipkin formerly had to
+// import this directly. A less precise name is better than rev-locking code.
 public class SamplerAutoConfiguration {
 
+	@Bean
+	@ConditionalOnMissingBean
+	Sampler sleuthTraceSampler() {
+		return Sampler.NEVER_SAMPLE;
+	}
+
 	static Sampler samplerFromProps(SamplerProperties config) {
 		if (config.getRate() != null) {
 			return new RateLimitingSampler(config);

spring-cloud-sleuth-core/src/main/resources/META-INF/spring.factories

@@ -1,7 +1,6 @@
 # Auto Configuration
 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
 org.springframework.cloud.sleuth.annotation.SleuthAnnotationAutoConfiguration,\
-org.springframework.cloud.sleuth.sampler.SamplerAutoConfiguration,\
 org.springframework.cloud.sleuth.autoconfig.TraceAutoConfiguration,\
 org.springframework.cloud.sleuth.log.SleuthLogAutoConfiguration,\
 org.springframework.cloud.sleuth.propagation.SleuthTagPropagationAutoConfiguration,\

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

@@ -36,7 +36,6 @@ import org.springframework.boot.context.properties.EnableConfigurationProperties
 import org.springframework.cloud.client.serviceregistry.Registration;
 import org.springframework.cloud.commons.util.InetUtils;
 import org.springframework.cloud.sleuth.autoconfig.TraceAutoConfiguration;
-import org.springframework.cloud.sleuth.sampler.SamplerAutoConfiguration;
 import org.springframework.cloud.sleuth.zipkin2.sender.ZipkinSenderConfigurationImportSelector;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -46,8 +45,7 @@ import org.springframework.web.client.RestTemplate;
 
 /**
  * {@link org.springframework.boot.autoconfigure.EnableAutoConfiguration
- * Auto-configuration} enables reporting to Zipkin via HTTP. Has a default sampler set
- * from the {@link SamplerAutoConfiguration}
+ * Auto-configuration} enables reporting to Zipkin via HTTP.
  *
  * The {@link ZipkinRestTemplateCustomizer} allows you to customize the
  * {@link RestTemplate} that is used to send Spans to Zipkin. Its default implementation -
@@ -56,7 +54,6 @@ import org.springframework.web.client.RestTemplate;
  * @author Spencer Gibb
  * @author Tim Ysewyn
  * @since 1.0.0
- * @see SamplerAutoConfiguration
  * @see ZipkinRestTemplateCustomizer
  * @see DefaultZipkinRestTemplateCustomizer
  */
@@ -66,7 +63,7 @@ import org.springframework.web.client.RestTemplate;
 		"spring.zipkin.enabled" }, matchIfMissing = true)
 @AutoConfigureBefore(TraceAutoConfiguration.class)
 @AutoConfigureAfter(name = "org.springframework.cloud.autoconfigure.RefreshAutoConfiguration")
-@Import({ ZipkinSenderConfigurationImportSelector.class, SamplerAutoConfiguration.class })
+@Import(ZipkinSenderConfigurationImportSelector.class)
 public class ZipkinAutoConfiguration {
 
 	/**