Fix service-entry spans inheriting gateway domain name as service name under inferred proxy spans

  When an inferred proxy span is the parent, the service-entry span was inheriting
  its service name (the gateway domain name, e.g. system-tests-api-gateway.com)
  instead of using the application's configured service name. Reset the service name
  to the configured DD_SERVICE value in HttpServerDecorator.startSpan() when an
  inferred proxy parent is detected.

Author: jandro996

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/bootstrap/instrumentation/decorator/HttpServerDecorator.java

@@ -178,6 +178,10 @@ public Context startSpan(REQUEST_CARRIER carrier, Context parentContext) {
     // finish calls from child spans (e.g., Spring MVC handler) are deferred until the
     // service-entry span finishes (after the response status is known).
     registerServiceEntrySpanInInferredProxy(parentContext, span);
+    // Reset service name inherited from inferred proxy parent: the inferred span uses the
+    // gateway domain name as service name, but the service-entry span should identify
+    // the application (configured DD_SERVICE), not the upstream gateway.
+    resetServiceNameIfUnderInferredProxy(parentContext, span);
     // Apply RequestBlockingAction if any
     Flow<Void> flow = callIGCallbackRequestHeaders(span, carrier);
     if (flow.getAction() instanceof RequestBlockingAction) {
@@ -205,6 +209,12 @@ private void registerServiceEntrySpanInInferredProxy(
     }
   }
 
+  private void resetServiceNameIfUnderInferredProxy(Context parentContext, AgentSpan span) {
+    if (InferredProxySpan.fromContext(parentContext) != null) {
+      span.setServiceName(Config.get().getServiceName());
+    }
+  }
+
   private final DataStreamsTransactionTracker.TransactionSourceReader
       DSM_TRANSACTION_SOURCE_READER =
           (source, headerName) -> {

dd-java-agent/instrumentation/spring/spring-webmvc/spring-webmvc-3.1/src/test/groovy/test/boot/SpringBootBasedTest.groovy

@@ -543,10 +543,8 @@ class SpringBootBasedTest extends HttpServerTest<ConfigurableApplicationContext>
           }
         }
         // Server span should be a child of the inferred proxy span
-        // When there's an inferred proxy span parent, the server span inherits the parent's service name
         span {
-          // Service name is inherited from the inferred proxy span parent
-          serviceName "api.example.com"
+          serviceName expectedServiceName()
           operationName operation()
           resourceName expectedResourceName(SUCCESS, "GET", address)
           spanType DDSpanTypes.HTTP_SERVER
@@ -570,9 +568,8 @@ class SpringBootBasedTest extends HttpServerTest<ConfigurableApplicationContext>
           }
         }
         if (hasHandlerSpan()) {
-          // Handler span inherits service name from inferred proxy span parent
           it.span {
-            serviceName "api.example.com"
+            serviceName expectedServiceName()
             operationName "spring.handler"
             resourceName "TestController.success"
             spanType DDSpanTypes.HTTP_SERVER
@@ -585,9 +582,8 @@ class SpringBootBasedTest extends HttpServerTest<ConfigurableApplicationContext>
             }
           }
         }
-        // Controller span also inherits service name
         it.span {
-          serviceName "api.example.com"
+          serviceName expectedServiceName()
           operationName "controller"
           resourceName "controller"
           errored false