code: @BetaApi
    public final RoutersPreviewResponse previewRouter(String router, Router routerResource) {
  
      PreviewRouterHttpRequest request =
          PreviewRouterHttpRequest.newBuilder()
              .setRouter(router)
              .setRouterResource(routerResource)
              .build();
      return previewRouter(request);
    }
docs: Preview fields auto-generated during router create and update operations. Calling this method
  does NOT create or update the router.
  
  <p>Sample code:
  
  <pre><code>
  try (RouterClient routerClient = RouterClient.create()) {
  ProjectRegionRouterName router = ProjectRegionRouterName.of("[PROJECT]", "[REGION]", "[ROUTER]");
  Router routerResource = Router.newBuilder().build();
  RoutersPreviewResponse response = routerClient.previewRouter(router.toString(), routerResource);
  }
  </code></pre>
  
  @param router Name of the Router resource to query.
  @param routerResource Router resource.
  @throws com.google.api.gax.rpc.ApiException if the remote call fails