Regenerate gmailpostmastertools client (#6655)

Generated in GitHub action: https://togithub.com/googleapis/googleapis/google-api-php-client-services/actions/workflows/codegen.yaml

Author: yoshi-code-bot

src/PostmasterTools.php

@@ -36,7 +36,9 @@
  */
 class PostmasterTools extends \Google\Service
 {
-
+  /** See email traffic metrics for the domains you have registered in Gmail Postmaster Tools. */
+  const POSTMASTER_READONLY =
+      "https://www.googleapis.com/auth/postmaster.readonly";
 
   public $domains;
   public $domains_trafficStats;