Add getRootSpan() to MutableSpan

Author: Andrew Kent

dd-trace-api/src/main/java/datadog/trace/api/interceptor/MutableSpan.java

@@ -41,4 +41,6 @@ public interface MutableSpan {
   Boolean isError();
 
   MutableSpan setError(boolean value);
+
+  MutableSpan getRootSpan();
 }

dd-trace-ot/src/main/java/datadog/opentracing/DDSpan.java

@@ -125,6 +125,11 @@ public DDSpan setError(final boolean error) {
     return this;
   }
 
+  @Override
+  public MutableSpan getRootSpan() {
+    return context().getTrace().getRootSpan();
+  }
+
   public void setErrorMeta(final Throwable error) {
     setError(true);
 

dd-trace-ot/src/test/groovy/datadog/opentracing/DDSpanTest.groovy

@@ -183,4 +183,18 @@ class DDSpanTest extends Specification {
     child2.getMetrics().get(DDSpanContext.PRIORITY_SAMPLING_KEY) == null
     child2.getMetrics().get(DDSpanContext.SAMPLE_RATE_KEY) == null
   }
+
+  def "getRootSpan returns the root span"() {
+    setup:
+    def root = tracer.buildSpan("root").start()
+    def child = tracer.buildSpan("child").asChildOf(root).start()
+
+    expect:
+    root.getRootSpan() == root
+    child.getRootSpan() == root
+
+    cleanup:
+    child.finish()
+    root.finish()
+  }
 }