Add opentracing example to set span

vertx-opentracing/src/main/asciidoc/index.adoc

@@ -17,3 +17,18 @@ over the configuration.
 ----
 {@link examples.OpenTracingExamples#ex2}
 ----
+
+After the Tracer has been configured, Vert.x may inject _spans_ into existing traces for
+some API calls, such as HTTP client or server requests. A trace that starts from a server
+call will be automatically created by Vert.x.
+
+However, to initiate a trace for a client call, you need to create it first and make Vert.x
+aware of it by using `OpenTracingUtil.setSpan`:
+
+[source,$lang]
+----
+{@link examples.OpenTracingExamples#ex3}
+----
+
+In an HTTP scenario between two Vert.x services, a span will be created client-side, then
+the trace context will be propagated server-side and another span will be added to the trace.

vertx-opentracing/src/main/java/examples/OpenTracingExamples.java

@@ -1,10 +1,12 @@
 package examples;
 
+import io.opentracing.Span;
 import io.opentracing.Tracer;
 import io.vertx.core.Vertx;
 import io.vertx.core.VertxOptions;
 import io.vertx.docgen.Source;
 import io.vertx.tracing.opentracing.OpenTracingOptions;
+import io.vertx.tracing.opentracing.OpenTracingUtil;
 
 @Source
 public class OpenTracingExamples {
@@ -24,4 +26,13 @@ public void ex2(Tracer tracer) {
       )
     );
   }
+
+  public void ex3(Tracer tracer) {
+    Span span = tracer.buildSpan("my-operation")
+      .withTag("some-key", "some-value")
+      .start();
+    OpenTracingUtil.setSpan(span);
+    // Do something, e.g. client request
+    span.finish();
+  }
 }