Can we do rulerMIMEType = tripperware.QueryResponseCortexMIMEType here?

The tripperware.QueryResponseCortexMIMEType is a string, so it is hard to use it to MIMEType directly.

Nit: Consider returning here vs else block

What do you mean is like this?

if isSourceRuler {
		// When the source is the Ruler, then forward whole headers
		result.Headers = r.Header
		return &result, nil
	} else {
		// Include the specified headers from http request in prometheusRequest.
		for _, header := range forwardHeaders {
			for h, hv := range r.Header {
				if strings.EqualFold(h, header) {
					result.Headers[h] = hv
					break
				}
			}
		}
	}

	return &result, nil

Something like this

if isSourceRuler {
		// When the source is the Ruler, then forward whole headers
		result.Headers = r.Header
		return &result, nil
	} 
// Include the specified headers from http request in prometheusRequest.
for _, header := range forwardHeaders {
	for h, hv := range r.Header {
		if strings.EqualFold(h, header) {
			result.Headers[h] = hv
			break
		}
	}
}
return &result, nil

Should we only try to return non JSON response format if we know the request comes from Ruler?
If the request doesn't come from Ruler I feel we want to enforce JSON response even though they specify protobuf in their protocol like Accept: protobuf, json. To reduce impact as this protobuf format is Cortex specific.

But tbh Idk if we can identify ruler requests or not. We probably have to do what you are doing here.

WDYT? @alanprot @danielblando

@yeya24
One way is to define a new MIME type like application/x-protobuf-ruler (just example) and then use it to accept header.

Yeah.. maybe we should define our internal MIME type so external calls will only get that format if they really wanna.
I would not add "ruler" in the format though. Maybe something like application/x-cortex.query.v1+proto (idk the latest best practices to define those custom Mime types :P )

This sounds worth trying. Let's hear from other maintainers, too