Fixes and backfills missing test of localEndpoint IP default (#1251)

When we switched to use zipkin-reporter-brave, we goofed IP defaults. It happened because we never tested existence of local IP all these years! Thanks to @heesuk-ahn for reporting

Author: adriancole

brave/src/main/java/brave/Tracing.java

@@ -475,6 +475,10 @@ static final class Default extends Tracing {
  this.noop = new AtomicBoolean();
 
  MutableSpan defaultSpan = new MutableSpan(builder.defaultSpan); // safe copy
+ // Lazy add localEndpoint.ip if not yet set
+ if (defaultSpan.localIp() == null) {
+ defaultSpan.localIp(Platform.get().linkLocalIp());
+ }
 
  Set<SpanHandler> spanHandlers = new LinkedHashSet<>(builder.spanHandlers);
  // When present, the Zipkin handler is invoked after the user-supplied ones.

brave/src/test/java/brave/TracingTest.java

@@ -16,6 +16,7 @@
 import brave.handler.FinishedSpanHandler;
 import brave.handler.MutableSpan;
 import brave.handler.SpanHandler;
+import brave.internal.Platform;
 import brave.propagation.B3SinglePropagation;
 import brave.propagation.Propagation;
 import brave.propagation.StrictCurrentTraceContext;
@@ -70,6 +71,14 @@ public class TracingTest {
  assertThat(spans).isEmpty();
  }
 
+ @Test public void localEndpointDefaults() {
+ Tracing tracing = Tracing.newBuilder().build();
+ assertThat(tracing).extracting("tracer.pendingSpans.defaultSpan.localServiceName")
+ .isEqualTo("unknown");
+ assertThat(tracing).extracting("tracer.pendingSpans.defaultSpan.localIp")
+ .isEqualTo(Platform.get().linkLocalIp());
+ }
+
  @Test public void localServiceNamePreservesCase() {
  String expectedLocalServiceName = "FavStar";
  Tracing.Builder builder = Tracing.newBuilder().localServiceName(expectedLocalServiceName);