(Docs) add missing js version for generateMetadata. (#52763)

There was no JS version for `generateMetadata` function.

fixes #52759 

Co-authored-by: Steven <229881+styfle@users.noreply.github.com>

docs/02-app/02-api-reference/04-functions/generate-metadata.mdx

@@ -109,6 +109,29 @@ export async function generateMetadata(
 export default function Page({ params, searchParams }: Props) {}
 ```
 
+```
+jsx filename="app/products/[id]/page.js" switcher
+export async function generateMetadata({ params, searchParams }, parent) {
+  // read route params
+  const id = params.id
+
+  // fetch data
+  const product = await fetch(`https://.../${id}`).then((res) => res.json())
+
+  // optionally access and extend (rather than replace) parent metadata
+  const previousImages = (await parent).openGraph?.images || []
+
+  return {
+    title: product.title,
+    openGraph: {
+      images: ['/some-specific-page-image.jpg', ...previousImages],
+    },
+  }
+}
+
+export default function Page({ params, searchParams }) {}
+```
+
 ### Parameters
 
 `generateMetadata` function accepts the following parameters: