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/plugins/network/index.js

@@ -107,6 +107,19 @@ export default class extends FlipperPlugin<State, *, PersistedState> {
     responses: {},
   };
 
+  static exportMetrics = (
+    persistedState: PersistedState,
+  ): Promise<Map<string, number | string>> => {
+    const failures = Object.keys(persistedState.responses).reduce(function(
+      previous,
+      key,
+    ) {
+      return previous + (persistedState.responses[key].status >= 400);
+    },
+    0);
+    return Promise.resolve(new Map([['NUMBER_NETWORK_FAILURES', failures]]));
+  };
+
   static persistedStateReducer = (
     persistedState: PersistedState,
     method: string,