Added docs with an example of CompletableFuture usage

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

@@ -487,6 +487,14 @@ If you want to skip Span creation for some `@Scheduled` annotated classes you ca
 We're providing `LazyTraceExecutor`, `TraceableExecutorService` and `TraceableScheduledExecutorService`. Those implementations
 are creating Spans each time a new task is submitted, invoked or scheduled.
 
+Here you can see an example of how to pass tracing information with `TraceableExecutorService` when working with `CompletableFuture`:
+
+[source,java]
+----
+
+include::../../../../spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/async/TraceableExecutorServiceTests.java[tags=completablefuture,indent=0]
+----
+
 === Messaging
 
 Spring Cloud Sleuth integrates with http://projects.spring.io/spring-integration/[Spring Integration]. It creates spans for publish and

spring-cloud-sleuth-core/src/test/java/org/springframework/cloud/sleuth/instrument/async/TraceableExecutorServiceTests.java

@@ -15,6 +15,7 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.mockito.Mock;
 import org.mockito.runners.MockitoJUnitRunner;
+import org.springframework.cloud.sleuth.DefaultSpanNamer;
 import org.springframework.cloud.sleuth.NoOpSpanReporter;
 import org.springframework.cloud.sleuth.Span;
 import org.springframework.cloud.sleuth.SpanNamer;
@@ -66,6 +67,25 @@ public class TraceableExecutorServiceTests {
 		then(this.spanVerifyingRunnable.spanIds.stream().distinct().collect(toList())).hasSize(TOTAL_THREADS);
 	}
 
+	@Test
+	public void should_propagate_trace_info_when_compleable_future_is_used() throws Exception {
+		Tracer tracer = this.tracer;
+		TraceKeys traceKeys = new TraceKeys();
+		SpanNamer spanNamer = new DefaultSpanNamer();
+
+		// tag::completablefuture[]
+		CompletableFuture<Long> completableFuture = CompletableFuture.supplyAsync(() -> {
+			// perform some logic
+			return 1_000_000L;
+		}, new TraceableExecutorService(Executors.newCachedThreadPool(),
+				// 'calculateTax' explicitly names the span - this param is optional
+				tracer, traceKeys, spanNamer, "calculateTax"));
+		// end::completablefuture[]
+
+		then(completableFuture.get()).isEqualTo(1_000_000L);
+		then(this.tracer.getCurrentSpan()).isNull();
+	}
+
 	private CompletableFuture<?>[] runnablesExecutedViaTraceManagerableExecutorService() {
 		List<CompletableFuture<?>> futures = new ArrayList<>();
 		for (int i = 0; i < TOTAL_THREADS; i++) {