Add new endpoint giving simple rustdoc status for a version

[Nemo157]

As discussed in #2144, this adds a new endpoint for simply requesting whether docs are available for a specific version, without exposing other details about our builds.

I've also fixed the metrics recording /builds.json requests as being "static resource" stopping us from being able to see how many requests we get, that will let us verify whether there's any other clients hitting that endpoint once shields.io/crates.io/lib.rs have migrated over.

[syphar]

Thank you for working on this!

[syphar]

Last two comments ;)

[syphar]

@Nemo157 after manually testing, I saw one thing:

we do seem to follow semver logic, so a request to /crate/itertools/0.10.0/status.json, where 0.10.0 doesn't exist, would lead to the endpoint returning the status for 0.10.5 (the latest semver for this).

Seeing the two use-cases I can imagine:

1. shields.io will request /latest/ (?)
2. crates.io will request a specific version

Perhaps I'm missing another case?

for the crates.io case I could imagine some edge-cases where the status for a version would be returned that crates.io didn't request. Though this could currently only happen when we're missing a version that is not the latest of that semver bracket.

Would crates.io check the returned version? Or should we perhaps return a redirect similar to the builds.json endpoint? ( crates.io would need to ignore that one too then).

[Nemo157]

Huh, interesting, likely the only reason that crates.io doesn't make that mistake right now is that the redirect is lacking a CORS header. Testing on https://crates.io/crates/stylish-core/0.1.0 (which isn't on docs.rs because of weird reasons around cyclic dependencies, but 0.1.1 is):

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://docs.rs/crate/stylish-core/0.1.0/builds.json. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Status code: 302.

I could make it only support exact or latest, and return an error on semver matches still, or when changing crates.io to use this endpoint make it do a request to /={version}/.

[syphar]

I could make it only support exact or latest, and return an error on semver matches still, or when changing crates.io to use this endpoint make it do a request to /={version}/.

Sleeping over it, expecting the latter sounds like a valid approach.

That means the only remaining question (for me, right now) seems to be:

so we want consistency with the other pages, and the builds.json endpoint? Because in these, we have redirects for semver matches.

[Nemo157]

@kornelski given lib.rs' source has gone private, would you be able to migrate to this new API? https://docs.rs/crate/{crate}/{version}/status.json returning a simple { doc_status: boolean, version: string } response. (The version parameter can be a semver matcher, if you want to do exact versioning like I'm recommending for crates.io you should use ={version}, but since you don't have per-version pages I'm not sure if that'd be the best approach).