I encountered this while looking into prometheus/prometheus#15777. It appears that a comma (,) is used as a separator in the Content-Type header instead of a semicolon (;).

here:

All definitions I've seen use a semicolon (;) and apparently the RFC that initially mentioned a comma has since published an errata.

More details here:
https://stackoverflow.com/questions/35761248/which-separator-should-be-used-in-the-content-type-header-for-a-multipart-data-r

Not sure why the other consumers are ok with it, maybe less strict parsing? Do you think this is something that can be adjusted?

By the way, we're looking for someone interested in contributing to the Prometheus Ruby Client, particularly for these changes, it revolves around parsers and content negotiation. Please let me know if you're interested.