I'm struggling to find documentation or examples of implementing an upload progress indicator using fetch.

This is the only reference I've found so far, which states:

Progress events are a high level feature that won't arrive in fetch for now. You can create your own by looking at the Content-Length header and using a pass-through stream to monitor the bytes received.

This means you can explicitly handle responses without a Content-Length differently. And of course, even if Content-Length is there it can be a lie. With streams you can handle these lies however you want.

How would I write "a pass-through stream to monitor the bytes" sent? If it makes any sort of difference, I'm trying to do this to power image uploads from the browser to Cloudinary.

NOTE: I am not interested in the Cloudinary JS library, as it depends on jQuery and my app does not. I'm only interested in the stream processing necessary to do this with native javascript and Github's fetch polyfill.

https://fetch.spec.whatwg.org/#fetch-api