Add version package

[SuperQ]

Migrate the version package from github.com/prometheus/common to github.com/prometheus/client_golang in order to break the circular dependency.

• Make version a top level package because it uses init() to populate data.

Related to: prometheus/common#58

[bwplotka]

LGTM, just I would put it prometheus/collectors/version to group collectors in some way.

[bwplotka]

In our client_golang sync, with @ArthurSens and @kakkoyun we decided at the end to move it to prometheus/collectors/version package. Hope @SuperQ that's ok, we want to do it before next release of client_golang.

Sorry for confusion. 🤗

[SuperQ]

I don't think that's the right place for it. The version package is mostly about having a structured version feature, the metrics collector is secondary.

[bwplotka]

Yes, I understand, but from the client_golang point of view it's generally metric collector. To use it you have provide that structure, which you might reuse in other places, but for this repo, this is the best place. One could provide this structure on their own and somewhere else, but it's too simple to split version structure AND collector separately, thus collectors directory might be a fair trade-off (:

I feel it's prometheus/collectors/version is still better than exporter_toolkit semantically. 🤔 But up to you! I think we decided to move it to prometheus/collectors/version for now, it's too confusing and disorganized to keep it in the root @ArthurSens will do the PR later on.

Sorry I indicated "fair" on Slack on your point around long URL, it's some argument but indeed pretty weak (was late for me) 🙈