add second docs.rs build process to metrics collection

[syphar]

after we got a bigger EC2 machine for docs.rs, we are setting up a second build-process to parallize builds.

This gives us a bigger advantage than just having faster non-parallel builds.

I don't know anything about the network topoligy between prometheus & the docs.rs server, so the change here is an untested guess.

The second build-server exposes a small HTTP server at the port where it returns the prometheus metrics.

If this doesn't work we could also configure a special path in the nginx config that proxies the request to the second build process.

[Mark-Simulacrum]

I checked the security group on the current docs.rs instance and it doesn't allow port 3001 through, so I think we will want to re-expose that through nginx.

(Already mentioned on Zulip but re-iterating here for a track record: https://rust-lang.zulipchat.com/#narrow/stream/356853-t-docs-rs/topic/second.20build.20process.20on.20server/near/396664285)

[syphar]

@Mark-Simulacrum I updated the NGINX config & this PR