Allow extending the metadata provider class

Packages/ohif-cornerstone-settings/imports/client/index.js

@@ -3,4 +3,7 @@ import { OHIF } from 'meteor/ohif:core';
 import './renderer.js';
 
 import { MetadataProvider } from './lib/classes/MetadataProvider';
+OHIF.cornerstone.MetadataProvider = MetadataProvider;
+
+// TODO: Remove this after all viewers are updated to create an instance of OHIF.cornerstone.MetadataProvider
 OHIF.cornerstone.metadataProvider = new MetadataProvider();