The ETagMiddleware generates a weak ETag from the response body. Re-request this URL with If-None-Match: W/"f532ffe7aa02b424d318568b15d1e33e" and you'll get a 304 Not Modified — body skipped, bandwidth saved.
GET /demo/middleware/etag W/"f532ffe7aa02b424d318568b15d1e33e"Try in DevTools → Network: reload, then reload again — second request returns 304.