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
afd729deb6