docs(migration): clarify on content-type headers

[LukoJy3D]

Previously, when creating a custom metric endpoint, we did not bother with setting the header, just making sure that the metric format met the standard. From the migration guide, it was unclear what format the content-type header is accepted in, and it is only visible in the source code. It would have helped me if I had found it sooner.

[machine424]

Thanks for this.
I think that instead of just listing the accepted protocols, which might lead users to choose the wrong one, it would be better to ensure that https://prometheus.io/docs/instrumenting/exposition_formats/ is as explicit and informative about the other formats as it is for the Text-based one. We can then simply link to that page in the migration guide.

There is #15830 to track that change, if you're interested.

(By the way, specifying the media type is sufficient, the version and other parameters are not required)

[bboreham]

Hello from the bug-scrub! @LukoJy3D would you like to make a PR for the https://prometheus.io/docs/instrumenting/exposition_formats/ page as suggested?

[LukoJy3D]

Hello, yes, let me look into this 👀

[LukoJy3D]

Landed a PR, yet while writing it felt like most of the info is already covered in https://prometheus.io/docs/instrumenting/content_negotiation/, so from my POV, while doing an upgrade I was kind of only missing some reference either to this page or some examples (including media type and not only the protocols)