Changed sampler method to be consistent with open tracing API

https://github.com/opentracing/opentracing-python/blob/master/example/zipkin_like/sampler.py#L36

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

@@ -20,5 +20,5 @@ package org.springframework.cloud.sleuth;
  * Extremely simple callback to determine the frequency that an action should be
  */
 public interface Sampler {
-	boolean next();
+	boolean isSampled();
 }

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

@@ -23,7 +23,7 @@ import org.springframework.cloud.sleuth.Sampler;
  */
 public class AlwaysSampler implements Sampler {
 	@Override
-	public boolean next() {
+	public boolean isSampled() {
 		return true;
 	}
 }

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

@@ -27,7 +27,7 @@ public class IsTracingSampler implements Sampler {
 	public static IsTracingSampler INSTANCE = new IsTracingSampler();
 
 	@Override
-	public boolean next() {
+	public boolean isSampled() {
 		return SpanContextHolder.isTracing();
 	}
 }

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

@@ -31,7 +31,7 @@ public class PercentageBasedSampler implements Sampler {
 	}
 
 	@Override
-	public boolean next() {
+	public boolean isSampled() {
 		Span currentSpan = this.traceAccessor.getCurrentSpan();
 		long threshold = Math.abs(Long.MAX_VALUE * (int) (this.configuration.getPercentage() * 100)); // drops fractional percentage.
 		if (currentSpan == null || threshold == 0L) {

spring-cloud-sleuth-core/src/main/java/org/springframework/cloud/sleuth/trace/DefaultTracer.java

@@ -72,7 +72,7 @@ public class DefaultTracer implements Tracer {
 	@Override
 	public Span startTrace(String name, Sampler s) {
 		Span span;
-		if (isTracing() || s.next()) {
+		if (isTracing() || s.isSampled()) {
 			span = createChild(getCurrentSpan(), name);
 		}
 		else {

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/web/TraceFilterTests.java

@@ -216,8 +216,8 @@ public class TraceFilterTests {
 
 	private class DelegateSampler implements Sampler {
 		@Override
-		public boolean next() {
-			return TraceFilterTests.this.sampler.next();
+		public boolean isSampled() {
+			return TraceFilterTests.this.sampler.isSampled();
 		}
 	}
 }

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/sampler/PercentageBasedSamplerTests.java

@@ -20,7 +20,7 @@ public class PercentageBasedSamplerTests {
 		this.samplerConfiguration.setPercentage(1f);
 
 		for (int i = 0; i < 10; i++) {
-			then(new PercentageBasedSampler(this.samplerConfiguration, this.traceAccessor).next()).isTrue();
+			then(new PercentageBasedSampler(this.samplerConfiguration, this.traceAccessor).isSampled()).isTrue();
 		}
 
 	}
@@ -30,7 +30,7 @@ public class PercentageBasedSamplerTests {
 		this.samplerConfiguration.setPercentage(0f);
 
 		for (int i = 0; i < 10; i++) {
-			then(new PercentageBasedSampler(this.samplerConfiguration, this.traceAccessor).next()).isFalse();
+			then(new PercentageBasedSampler(this.samplerConfiguration, this.traceAccessor).isSampled()).isFalse();
 		}
 	}
 
@@ -48,7 +48,7 @@ public class PercentageBasedSamplerTests {
 	private int countNumberOfSampledElements(int numberOfIterations) {
 		int passedCounter = 0;
 		for (int i = 0; i < numberOfIterations; i++) {
-			boolean passed = new PercentageBasedSampler(this.samplerConfiguration, traceReturningSpanWithUuid()).next();
+			boolean passed = new PercentageBasedSampler(this.samplerConfiguration, traceReturningSpanWithUuid()).isSampled();
 			passedCounter = passedCounter + (passed ? 1 : 0);
 		}
 		return passedCounter;