Support custom metro ports (#2050)

Summary:
Based on issue https://github.com/facebook/flipper/issues/1509 I needed a custom port for the metro server to pass to flipper.

## Changelog

introduce `METRO_SERVER_PORT` env variable to be able to customize default 8081 port on startup.

Pull Request resolved: https://github.com/facebook/flipper/pull/2050

Test Plan:
Tested with a locally built linux desktop app, this seems to work OK pour hermes debugging & RN logging.

React DevTools don't seem to find the running app, maybe there's a mapping to handle there too ?

Reviewed By: jknoxville

Differential Revision: D27339006

Pulled By: passy

fbshipit-source-id: b1700c4fe73f14bf4617e23583b2954012e0a5aa

desktop/plugins/hermesdebuggerrn/index.tsx

@@ -23,7 +23,10 @@ import ErrorScreen from './ErrorScreen';
 import ChromeDevTools from './ChromeDevTools';
 
 const POLL_SECS = 5 * 1000;
-const METRO_HOST = 'http://localhost:8081';
+const METRO_PORT_ENV_VAR = process.env.METRO_SERVER_PORT || '8081';
+const METRO_PORT = isNaN(+METRO_PORT_ENV_VAR) ? '8081' : METRO_PORT_ENV_VAR;
+const METRO_URL = new URL('http://localhost');
+METRO_URL.port = METRO_PORT;
 
 export type Target = Readonly<{
   id: string;
@@ -87,7 +90,7 @@ export default class extends FlipperDevicePlugin<State, any, any> {
   }
 
   checkDebugTargets = () => {
-    fetch(`${METRO_HOST}/json`)
+    fetch(`${METRO_URL.toString()}json`)
       .then((res) => res.json())
       .then(
         (result) => {