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

android/sample/src/main/java/com/facebook/flipper/sample/ExampleActions.java

@@ -46,8 +46,7 @@ public final class ExampleActions {
   }
 
   public static void sendGetRequest() {
-    final Request request =
-        new Request.Builder().url("https://api.github.com/repos/facebook/yoga").get().build();
+    final Request request = new Request.Builder().url("https://api.github.com/repos/facebook/yoga").get().build();
     FlipperSampleApplication.sOkHttpClient
         .newCall(request)
         .enqueue(