Expose MetroConfig type in metro package

Summary:
Pairs with D44416426. Slightly simplifies user setup when we decide to use this.

```diff
- /** type {import('metro-config').MetroConfig} */
+ /** type {import('metro').MetroConfig} */
  const config = { ... };
```

Changelog: **[Types]** Expose `MetroConfig` type in `metro` package

Reviewed By: motiz88

Differential Revision: D44540077

fbshipit-source-id: 03fa91ffcdc054cb898a7739680366d7d0b31217

Author: huntie

packages/metro/src/index.flow.js

@@ -20,6 +20,7 @@ import type {Server as HttpsServer} from 'https';
 import type {
   ConfigT,
   InputConfigT,
+  MetroConfig,
   Middleware,
 } from 'metro-config/src/configTypes.flow';
 import type {CustomTransformOptions} from 'metro-transform-worker';
@@ -120,6 +121,8 @@ export type RunBuildOptions = {
 type BuildCommandOptions = {} | null;
 type ServeCommandOptions = {} | null;
 
+export type {MetroConfig};
+
 async function getConfig(config: InputConfigT): Promise<ConfigT> {
   const defaultConfig = await getDefaultConfig(config.projectRoot);
   return mergeConfig(defaultConfig, config);

packages/metro/types/index.d.ts

@@ -16,7 +16,13 @@ export * from './lib/reporting';
 
 import type {Server as HttpServer} from 'http';
 import type {Server as HttpsServer} from 'https';
-import type {ConfigT, InputConfigT, loadConfig, Middleware} from 'metro-config';
+import type {
+  ConfigT,
+  InputConfigT,
+  loadConfig,
+  MetroConfig,
+  Middleware,
+} from 'metro-config';
 import type {CustomTransformOptions} from 'metro-babel-transformer';
 import type {ReadOnlyGraph} from './DeltaBundler/types';
 import type {Server} from 'ws';
@@ -101,6 +107,8 @@ interface BuildGraphOptions {
   type?: 'module' | 'script';
 }
 
+export {MetroConfig};
+
 export function runMetro(
   config: InputConfigT,
   options?: RunMetroOptions,