Add global hook to assert that base Metro config is called (#38126)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38126

Towards https://github.com/react-native-community/cli/issues/1987. Will be paired with a CLI PR targeting React Native 0.72.1.

Changelog: None

Reviewed By: motiz88

Differential Revision: D47125080

fbshipit-source-id: b3b9d93ba747240f5168021ccb793ffe5d34251d

packages/metro-config/index.js

@@ -77,6 +77,9 @@ function getDefaultConfig(
     watchFolders: [],
   };
 
+  // Set global hook so that the CLI can detect when this config has been loaded
+  global.__REACT_NATIVE_METRO_CONFIG_LOADED = true;
+
   return mergeConfig(
     getBaseConfig.getDefaultValues(projectRoot),
     config,