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/utils/exportMetrics.js

@@ -0,0 +1,52 @@
+/**
+ * Copyright 2018-present Facebook.
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ * @format
+ */
+import type {MiddlewareAPI} from '../reducers';
+import {serialize} from './serialization.js';
+import type {FlipperPlugin, FlipperDevicePlugin} from 'flipper';
+
+type MetricType = Map<string, Map<string, string | number>>;
+
+export type ExportMetricType = Map<string, MetricType>;
+
+export default async function exportMetrics(
+  store: MiddlewareAPI,
+): Promise<string> {
+  const state = store.getState();
+  let metrics: ExportMetricType = new Map();
+  for (let key in state.pluginStates) {
+    const pluginStateData = state.pluginStates[key];
+    const arr = key.split('#');
+    const pluginName = arr.pop();
+    const clientID = arr.join('#');
+    const pluginsMap: Map<
+      string,
+      Class<FlipperDevicePlugin<> | FlipperPlugin<>>,
+    > = new Map([]);
+    state.plugins.clientPlugins.forEach((val, key) => {
+      pluginsMap.set(key, val);
+    });
+    state.plugins.devicePlugins.forEach((val, key) => {
+      pluginsMap.set(key, val);
+    });
+    const exportMetrics1: ?(
+      persistedState: any,
+    ) => Promise<Map<string, string | number>> = pluginsMap.get(pluginName)
+      ?.exportMetrics;
+    if (pluginsMap.has(pluginName) && exportMetrics1) {
+      const metricMap = await exportMetrics1(pluginStateData);
+      const pluginMap = new Map([[pluginName, metricMap]]);
+      if (!metrics.get(clientID)) {
+        metrics.set(clientID, pluginMap);
+        continue;
+      }
+      const prevMetricMap = metrics.get(clientID);
+      // $FlowFixMe: prevMetricMap cannot be null, because clientID is added only when the pluingMetricMap is available
+      metrics.set(clientID, new Map([...prevMetricMap, ...pluginMap]));
+    }
+  }
+  return Promise.resolve(serialize(metrics));
+}