feat: expose watch option (#889)

Summary:
- Metro uses a pretty broad check to disable HMR. This PR exposes the `watch` option so Expo CLI can manually enable/disable file watching, then log to users that HMR is disabled.
- Related expo/expo-cli#4418 (comment)

Pull Request resolved: #889

Test Plan: - flow types

Reviewed By: huntie

Differential Revision: D41081944

Pulled By: jacdebug

fbshipit-source-id: 5f1a0d5c28617eee6f076baee299f7f7c4dd6ca3

packages/metro/src/index.flow.js

@@ -65,6 +65,7 @@ export type RunServerOptions = {
   secureCert?: string, // deprecated
   secureKey?: string, // deprecated
   waitForBundler?: boolean,
+  watch?: boolean,
   websocketEndpoints?: {
     [path: string]: typeof ws.Server,
   },
@@ -225,6 +226,7 @@ exports.runServer = async (
     secureKey, // deprecated
     waitForBundler = false,
     websocketEndpoints = {},
+    watch,
   }: RunServerOptions,
 ): Promise<HttpServer | HttpsServer> => {
   await earlyPortCheck(host, config.server.port);
@@ -246,6 +248,7 @@ exports.runServer = async (
   const {middleware, end, metroServer} = await createConnectMiddleware(config, {
     hasReducedPerformance,
     waitForBundler,
+    watch,
   });
 
   serverApp.use(middleware);