SmallRye Metrics should respond with Accept All headers

[kenfinnigan]

Describe the bug
Metrics are not returned when accessing /q/metrics

Expected behavior
If no Accept header is passed, ie. it means Accept All, then the metrics should be returned

Actual behavior
Currently, SmallRye Metrics returns a HTTP 406 if text/plain is not passed as an Accept header type.

[quarkus-bot]

/cc @Ladicek, @jmartisk, @phillip-kruger, @radcortez

[jmartisk]

I'm not sure what you mean here - if I call simply curl localhost:8080/q/metrics without any headers, I get a Prometheus export by default. I get a 406 error only if I pass an Accept header with an unrecognised value.

[kenfinnigan]

@ebullient added a test yesterday where we had an issue where the behavior you describe didn't happen.

I don't believe the test was adding any accept headers to the request

[ebullient]

I'll double check. this also may not be something unique to smallrye metrics, as I have #15093 also. I'll poke so we can be more precise about where something might need to be adjusted.