Refactor headless start function

Summary: Refactors the headless to have a list of closures instead of the `if` conditions. This will make addition of new arguments easy as it will be just adding a new closure to the list. One can change the order of the execution of the closures by inserting it at whatever index the user wants.

Reviewed By: passy

Differential Revision: D15965142

fbshipit-source-id: e9f348fe9d9011adcd47d140713d6187eab3b3d3

src/utils/exportMetrics.js

@@ -76,7 +76,7 @@ function parseJSON(str: string): ?any {
 export async function exportMetricsFromTrace(
   trace: string,
   pluginsMap: Map<string, Class<FlipperDevicePlugin<> | FlipperPlugin<>>>,
-): Promise<?string> {
+): Promise<string> {
   const data = fs.readFileSync(trace, 'utf8');
   const parsedJSONData = parseJSON(data);
   if (!parsedJSONData) {