add waiting hook in HttpServerTest to try to solve Pekko's flakiness (#9493)

Author: vandonr

dd-java-agent/instrumentation-testing/src/main/groovy/datadog/trace/agent/test/base/HttpServerTest.groovy

@@ -174,6 +174,15 @@ abstract class HttpServerTest<SERVER> extends WithHttpServer<SERVER> {
     }
   }
 
+  /**
+   * Hook method to allow subclasses to wait for request processing to complete.
+   * This is particularly useful for asynchronous HTTP server implementations
+   * where spans may still not be closed after the HTTP response is sent.
+   */
+  protected void waitForRequestToComplete() {
+    // Default implementation does nothing - override in subclasses as needed
+  }
+
   // used in blocking tests to check if the handler was skipped
   volatile boolean handlerRan
 
@@ -1195,6 +1204,8 @@ abstract class HttpServerTest<SERVER> extends WithHttpServer<SERVER> {
       TEST_DATA_STREAMS_WRITER.waitForGroups(1)
     }
 
+    waitForRequestToComplete()
+
     expect:
     response.code() == EXCEPTION.status
     if (testExceptionBody()) {

dd-java-agent/instrumentation/pekko-http-1.0/src/baseTest/groovy/PekkoHttpServerInstrumentationTest.groovy

@@ -40,6 +40,21 @@ abstract class PekkoHttpServerInstrumentationTest extends HttpServerTest<PekkoHt
     true
   }
 
+  @Override
+  protected void waitForRequestToComplete() {
+    // Pekko HTTP is asynchronous, and spans may not be completed
+    // immediately after the HTTP response is sent. Wait for the trace
+    // to be written to avoid race conditions in trace assertions.
+    try {
+      // Wait up to 1 sec for the trace to be written
+      TEST_WRITER.waitForTracesMax(1, 1)
+    } catch (InterruptedException e) {
+      Thread.currentThread().interrupt()
+      // If interrupted, proceed anyway - test may still pass
+    }
+  }
+
+
   //@Ignore("https://github.com/DataDog/dd-trace-java/pull/5213")
   @Override
   boolean testBadUrl() {