Add unstable_extraMiddleware option to runServer

Summary:
This adds a new option to Metro's  `runServer` API providing a convenient hook to mount custom [`connect`](https://www.npmjs.com/package/connect) middleware on Metro's HTTP server. This replaces the `server.enhanceMiddleware` Metro config option (deprecated in D46356577).

This option is `unstable_` as we work through using this option in integrating projects.

Changelog: **[Experimental]** Add `unstable_extraMiddleware` option to `runServer` API

Reviewed By: motiz88

Differential Revision: D46359829

fbshipit-source-id: 58aafcf1f33aed24a5163f8f4277976bff3717f4

docs/Configuration.md

@@ -612,7 +612,7 @@ Type: `(Middleware, MetroServer) => Middleware`
 A function that allows attaching custom [`connect`](https://www.npmjs.com/package/connect) middleware to Metro. For example:
 
 :::tip
-You can use [`connect()`](https://www.npmjs.com/package/connect#mount-middleware) as a util to extend the base `metroMiddleware` and to mount additional middleware handlers.
+You can use [`connect()`](https://www.npmjs.com/package/connect#mount-middleware) as a utility to extend the base `metroMiddleware` and to mount additional middleware handlers.
 :::
 
 ```ts

packages/metro/src/index.flow.js

@@ -11,6 +11,7 @@
 
 'use strict';
 
+import type {HandleFunction} from 'connect';
 import type {CustomResolverOptions} from 'metro-resolver';
 import type {ReadOnlyGraph} from './DeltaBundler';
 import type {ServerOptions} from './Server';
@@ -82,6 +83,7 @@ export type RunServerOptions = $ReadOnly<{
   secure?: boolean, // deprecated
   secureCert?: string, // deprecated
   secureKey?: string, // deprecated
+  unstable_extraMiddleware?: $ReadOnlyArray<HandleFunction>,
   waitForBundler?: boolean,
   watch?: boolean,
   websocketEndpoints?: $ReadOnly<{
@@ -254,6 +256,7 @@ exports.runServer = async (
     secure, //deprecated
     secureCert, // deprecated
     secureKey, // deprecated
+    unstable_extraMiddleware,
     waitForBundler = false,
     websocketEndpoints = {},
     watch,
@@ -283,6 +286,10 @@ exports.runServer = async (
 
   serverApp.use(middleware);
 
+  for (const handler of unstable_extraMiddleware ?? []) {
+    serverApp.use(handler);
+  }
+
   let inspectorProxy: ?InspectorProxy = null;
   if (config.server.runInspectorProxy) {
     inspectorProxy = new InspectorProxy(config.projectRoot);

packages/metro/types/index.d.ts

@@ -14,6 +14,7 @@ export * from './ModuleGraph/worker/collectDependencies';
 export * from './Server';
 export * from './lib/reporting';
 
+import type {HandleFunction} from 'connect';
 import type {EventEmitter} from 'events';
 import type {IncomingMessage, Server as HttpServer} from 'http';
 import type {Server as HttpsServer} from 'https';
@@ -72,6 +73,7 @@ export interface RunServerOptions {
   /** @deprecated since version 0.61 */
   secureKey?: string;
 
+  unstable_extraMiddleware?: ReadonlyArray<HandleFunction>;
   waitForBundler?: boolean;
   watch?: boolean;
   websocketEndpoints?: {