Returned Content-Type is malformed

Realized this while looking into https://github.com/prometheus/prometheus/issues/15777

The exporter is sending a Content-Type with a , as a separator instead of a semicolon ; as shown here !71 (comment 88627545) as well.

I opened https://github.com/sinatra/sinatra/issues/2076 for sinatra.

Not sure if there's a workaround on the exporter side while waiting for a fix in sinatra, but we can keep this issue open for informational and tracking purposes.

EDIT: apparently hardcoding the charset as in !211 (merged) may help.

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Also, 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.