Add Span to Sampler method params

Also allows us to actually create a PercentageBasedSampler (I
don't think anyone tried it before) without resorting to
lazy beans and proxies.

Another freature added here is a default percentage sampler
if we know that spans need to be exported (zipkin or stream
is present).

Fixes gh-138

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-zipkin/src/main/java/sample/SampleZipkinApplication.java

@@ -16,17 +16,16 @@
 
 package sample;
 
-import lombok.extern.slf4j.Slf4j;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
-import org.springframework.cloud.sleuth.Sampler;
-import org.springframework.cloud.sleuth.sampler.AlwaysSampler;
 import org.springframework.cloud.sleuth.zipkin.ZipkinSpanReporter;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.EnableAspectJAutoProxy;
 import org.springframework.scheduling.annotation.EnableAsync;
 
+import lombok.extern.slf4j.Slf4j;
+
 /**
  * @author Spencer Gibb
  */
@@ -36,14 +35,6 @@ import org.springframework.scheduling.annotation.EnableAsync;
 @Slf4j
 public class SampleZipkinApplication {
 
-	/**
-	 * Sleuth will not report trace data unless you define a sampler like below.
-	 */
-	@Bean
-	public Sampler defaultSampler() {
-		return new AlwaysSampler();
-	}
-
 	public static void main(String[] args) {
 		SpringApplication.run(SampleZipkinApplication.class, args);
 	}

spring-cloud-sleuth-samples/spring-cloud-sleuth-sample-zipkin/src/main/resources/application.yml

@@ -12,4 +12,4 @@ spring:
 sample:
   zipkin:
     # When enabled=false, traces log to the console. Comment to send to zipkin
-    enabled: true
+    enabled: false