Added metrics argument to headless inorder to export metrics

Summary:
This diff makes headless flipper accept `metrics` argument. Once this is passed. The headless flipper will terminate to export the metrics of the plugins, the type of the export looks like the following.

```
export type MetricType = {[metricName: string]: number};
type MetricPluginType = {[pluginID: string]: MetricType};
export type ExportMetricType = {[clientID: string]: MetricPluginType};

```

This diff, uses the store to export the metrics. I will modify the logic to accept the data which gets exported through `exportData`

Reviewed By: passy

Differential Revision: D14933499

fbshipit-source-id: dade5b7bc59ea4beb6d16c5ef471737e8597358a

src/plugin.js

@@ -83,6 +83,9 @@ export class FlipperBasePlugin<
     method: string,
     data: Object,
   ) => $Shape<PersistedState>;
+  static exportMetrics: ?(
+    persistedState: PersistedState,
+  ) => Promise<Map<string, number | string>>;
   static exportPersistedState: ?(
     callClient: (string, ?Object) => Promise<Object>,
     persistedState: ?PersistedState,