Add global hook to assert that base Metro config is called

Summary:
Towards react-native-community/cli#1987. Will be paired with a CLI PR targeting React Native 0.72.1.

Changelog: None

Differential Revision: D47125080

fbshipit-source-id: 17dec929313605fe7a91b1b5c6923ee04bdcf8b9

Author: huntie

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,