## How to configure HAProxy to support end-to-end (e2e) connections that switch from h2 to h2c

This was tested to work using HAProxy 2.8 on Ubuntu 20.04.

I'm not sure if `mode http` is a necessary line or not.

### Use case

This lets you run a cluster of API servers running Fastify, configured to use HTTP/2 (`{ http2: true }`), behind a HAProxy LB, so that you can serve typical REST API requests as well as SSE (Server Side Events) streaming connections, so that you can do fancy real-time page updates in your app.

Using HTTP/2 resolves the "6 concurrent HTTP connections" limit imposed by Chrome/Firefox/etc, which means the web app would become unresponsive if the user has 6+ tabs open with the app simultaneously.