Fix the export metrics format

Summary: Fix the format of the export. Due to unknown reason the changes done in the D14933499 was [reverted back](https://our.intern.facebook.com/intern/diff/D14933499/?src_number=80619316) on the same diff. Thus this diff fixes the format of the metrics export

Reviewed By: passy

Differential Revision: D15239653

fbshipit-source-id: b9416f89e1b0022000d6201eb08ca1f456d5ed2e

src/index.js

@@ -17,6 +17,7 @@ export {
   callClient,
 } from './plugin.js';
 export type {PluginClient, Props} from './plugin.js';
+export type {MetricType} from './utils/exportMetrics.js';
 export {default as Client} from './Client.js';
 export {clipboard} from 'electron';
 export * from './fb-stubs/constants.js';