Don't trace httpurlconnection as a top-level trace

dd-java-agent/instrumentation/http-url-connection/src/main/java/datadog/trace/instrumentation/http_url_connection/HttpUrlConnectionInstrumentation.java

@@ -107,12 +107,17 @@ public class HttpUrlConnectionInstrumentation extends Instrumenter.Default {
         return null;
       }
 
+      final Tracer tracer = GlobalTracer.get();
+      if (tracer.activeSpan() == null) {
+        // httpurlconnection doesn't play nicely with top-level spans
+        return null;
+      }
+
       final int callDepth = CallDepthThreadLocalMap.incrementCallDepth(HttpURLConnection.class);
       if (callDepth > 0) {
         return null;
       }
 
-      final Tracer tracer = GlobalTracer.get();
       final Scope scope =
           tracer
               .buildSpan("http.request")

dd-java-agent/instrumentation/http-url-connection/src/test/groovy/HttpUrlConnectionTest.groovy

@@ -332,6 +332,20 @@ class HttpUrlConnectionTest extends AgentTestRunner {
     }
   }
 
+  def "top level httpurlconnection tracing disabled"() {
+    setup:
+    HttpURLConnection connection = server.address.toURL().openConnection()
+    connection.addRequestProperty("is-dd-server", "false")
+    def stream = connection.inputStream
+    def lines = stream.readLines()
+    stream.close()
+    assert connection.getResponseCode() == STATUS
+    assert lines == [RESPONSE]
+
+    expect:
+    assertTraces(TEST_WRITER, 0) {}
+  }
+
   def "rest template"() {
     setup:
     runUnderTrace("someTrace") {