Info annotations from responses are not exposed

[laurynaslubys]

Prometheus somewhat recently added info annotations as a separate field, however this is not exposed via client_golang. Exposing them without changing or expanding the public API seems not possible.

What would be a good approach to take here?

[bwplotka]

Thanks!

Indeed we have to likely move Warnings to Annotations type like:

// Ref: https://github.com/prometheus/prometheus/pull/14429/files
type Annotations struct{
    Infos []string
    Warnings []string
}

We have a couple of options.

1. Technically speaking we could break API. API is kind of an experimental version, although it's obviously not very explicit since it's part of the v1 semver module.
2. We could deprecate the current API and move it (with necessary changes and breakage) to the upcoming exp module, so we can explicitly break an API. While annoying, it would be at least more expected for callers, when they compiler fail to build after the upgrade. Move to another module was motivated here: proposal: Move API code to a separate module/repo #1649
3. We can do the same but to separate module that could have proper major version bumps. Trickier to maintain ofc (more modules, more releases) I don't know if we have headcount for this.
4. We played with the idea of deprecating the API and trying to build OpenAPI spec and generate Go code from it somewhere else, won't help with breakages I believe Migrate Prometheus APIs to OpenAPI/Swagger prometheus#2392

I think I would vote for 1 or 2

cc @kakkoyun @ArthurSens @vesari thoughts?