Test errors 8.x-1.x branch with Drupal 9.2 "Attempt to create a field field_metatags..."

I'm not sure, if this is actually issue with Metatag or Drupal core.
But if we clear field definitions cache after adding the field in the MetatagTokenTest::setUp(), it solves the problem with tests:

diff --git a/tests/src/Functional/MetatagTokenTest.php b/tests/src/Functional/MetatagTokenTest.php
index bf81357..554f565 100644
--- a/tests/src/Functional/MetatagTokenTest.php
+++ b/tests/src/Functional/MetatagTokenTest.php
@@ -46,6 +46,7 @@ class MetatagTokenTest extends BrowserTestBase {
 
     $this->drupalLogin($this->rootUser);
     $this->fieldUIAddNewField('/admin/config/people/accounts', 'metatags', 'Metatags', 'metatag');
+    $this->container->get('entity_field.manager')->clearCachedFieldDefinitions();
   }
 
   /**

I found that the problem arised after these changes in core/tests/Drupal/Tests/BrowserTestBase.php

@@ -556,11 +557,28 @@ public function installDrupal() {
     $this->prepareSettings();
     $this->doInstall();
     $this->initSettings();
-    $container = $this->initKernel(\Drupal::request());
+    $this->container = $container = $this->initKernel(\Drupal::request());
     $this->initConfig($container);
     $this->installDefaultThemeFromClassProperty($container);
     $this->installModulesFromClassProperty($container);
-    $this->rebuildAll();
+
+    // Clear the static cache so that subsequent cache invalidations will work
+    // as expected.
+    $this->container->get('cache_tags.invalidator')->resetChecksums();
+
+    // Set the dummy query string added to all CSS and JavaScript files.
+    // @todo Remove in https://www.drupal.org/project/drupal/issues/3207893.
+    _drupal_flush_css_js();
+
+    // Generate a route to prime the url generator with the correct base url.
+    // @todo Remove in https://www.drupal.org/project/drupal/issues/3207896.
+    Url::fromRoute('<front>')->setAbsolute()->toString();
+
+    // Explicitly call register() again on the container registered in \Drupal.
+    // @todo This should already be called through
+    //   DrupalKernel::prepareLegacyRequest() -> DrupalKernel::boot() but that
+    //   appears to be calling a different container.
+    $this->container->get('stream_wrapper_manager')->register();
   }

If I get back $this->rebuildAll(); the errors disappear

And in the last comment of 2189411 it says

I think there is the possibility of disruption to contrib tests here, so erring on the side of caution and leaving it as 9.2 only at this point rather than backporting.

Now I think source of the problem is outside the Metatag module. Because tests in MetatagTokenTest calls Drupal\Tests\field_ui\Traits\FieldUiTestTrait::fieldUIAddNewField() and next Drupal\Tests\user\Traits\UserCreationTrait::createUser() cause a problem.

I agree, the comment here is useful. Thank you @DamienMcKenna for your attention to the problem and quick response.