According to this comment on OTel's Prometheus exporter expfmt and promhttp are returning the wrong format if openmetrics-text 1.0.0 is requested. It seems openmetrics-text 1.0.0 is not supported only openmetrics-text 0.0.1 is.

If a user requests openmetrics-text 1.0.0:

Accept: application/openmetrics-text; version=1.0.0; charset=utf-8

The expected result should be in the OpenMetrics format, also the content-type should be:

Content-Type: application/openmetrics-text; version=1.0.0; charset=utf-8

But the result is in Prometheus plaintext format, the content-type says the same:

Content-Type: text/plain; version=0.0.4; charset=utf-8

If a user requests openmetrics-text 0.0.1:

Accept: application/openmetrics-text; version=0.0.1; charset=utf-8

The result is in the expected OpenMetrics format:

Content-Type: application/openmetrics-text; version=0.0.1; charset=utf-8

As far as I know, 0.0.1 should not be used, only 1.0.0.
According to the OpenMetrics specs:

The content type MUST be: application/openmetrics-text; version=1.0.0; charset=utf-8

Also the Prometheus server uses 1.0.0 for scraping: prometheus/prometheus#9430