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/app/src/utils/adbConfig.tsx

@@ -7,13 +7,13 @@
  * @format
  */
 
+import {parseEnvironmentVariableAsNumber} from './environmentVariables';
+
 export default () => {
-  const serverPortString = process.env.ANDROID_ADB_SERVER_PORT;
-  let port = 5037;
-  if (serverPortString) {
-    const parsedInt = parseInt(serverPortString, 10);
-    port = isNaN(parsedInt) ? port : parsedInt;
-  }
+  let port = parseEnvironmentVariableAsNumber(
+    'ANDROID_ADB_SERVER_PORT',
+    5037,
+  ) as number;
 
   let host = 'localhost';
 

desktop/app/src/utils/environmentVariables.tsx

@@ -25,3 +25,15 @@ export function parseFlipperPorts(
     };
   }
 }
+
+export function parseEnvironmentVariableAsNumber(
+  envVarName: string,
+  defaultValue?: number,
+): number | undefined {
+  const envVarAsString = process.env[envVarName];
+  if (envVarAsString) {
+    const parsedInt = parseInt(envVarAsString, 10);
+    return isNaN(parsedInt) ? defaultValue : parsedInt;
+  }
+  return defaultValue;
+}