Add integration tests for xDS flow control

Signed-off-by: hanbingleixue <[email protected]>

sermant-integration-tests/xds-service-test/spring-server/src/main/java/io/sermant/demo/spring/server/flowcontrol/RetryController.java

@@ -98,7 +98,7 @@ public ResponseEntity<String> testTimeOut() {
         try {
             Thread.sleep(timeout);
         } catch (InterruptedException e) {
-            return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body(String.valueOf(requestCount));
+            return ResponseEntity.status(HttpStatus.BAD_GATEWAY).body(String.valueOf(requestCount));
         }
         return ResponseEntity.ok().body(String.valueOf(requestCount));
     }