Docs: Composing middleware that forward request headers via `NextResponse.next({ request: { headers } })`

What is the documentation issue?

The Middleware documentation explains how to modify request headers using NextResponse.next({ request: { headers } }), but it doesn't cover how to compose multiple middleware that each need to forward request headers.

When using multiple middleware (e.g., authentication + i18n), each may need to forward internal request headers:

• Auth middleware forwards x-session-id, x-user-id
• i18n middleware forwards x-locale

There's no documented pattern for merging these forwarded headers. The current examples show single middleware only. This leads developers to either:

1. Mutate request.headers directly (not documented/supported)
2. Parse internal x-middleware-* headers (brittle)
3. Give up on composition

The docs should include a recommended pattern for this common use case.

Is there any context that might help us understand?

Example use case:

// auth-middleware.ts
export function withAuth(request) {
  const headers = new Headers(request.headers);
  headers.set('x-session-id', 'abc');
  return NextResponse.next({ request: { headers } });
}

// i18n-middleware.ts  
export function withLocale(request) {
  const headers = new Headers(request.headers);
  headers.set('x-locale', 'en');
  return NextResponse.next({ request: { headers } });
}

// middleware.ts - How to compose these?
export function middleware(request) {
  const authRes = withAuth(request);
  const i18nRes = withLocale(request);
  
  // ❌ No documented way to merge forwarded request headers
  // Currently must choose: authRes OR i18nRes, not both
}

Current workarounds:

• Mutating request.headers before calling other middleware (works but not documented/guaranteed)
• Parsing internal x-middleware-override-headers (brittle, depends on implementation details)

Why this matters:

• Middleware composition is essential for real apps
• Auth + i18n is a classic combination
• The lack of a pattern pushes complexity to userland
• Library authors (AuthKit, next-intl, etc.) can't provide clean composition helpers

Requested addition:
Document a recommended pattern for merging forwarded request headers, or note that this is a current limitation and suggest the least-brittle workaround.

Does the docs page already exist? Please link to it.

https://nextjs.org/docs/app/building-your-application/routing/middleware#setting-headers

The "Setting Headers" section shows the basic usage but doesn't address composition.