fix: rt: in docs.rs, annotate the module as requiring metrics-rs-integration along with rt and tokio_unstable

[jlizen]

Still need to test locally - just was getting the PR open before closing up for the day :)

[jlizen]

Re: testing, i see attributes defined on the items both in their root crate export: https://github.com/tokio-rs/tokio-metrics/blob/main/src/lib.rs#L165 as well as on their direct declaration: https://github.com/tokio-rs/tokio-metrics/blob/main/src/runtime.rs#L8

So wanted to double check whether both are required, which one, etc.

If it's needed everywhere, I can take a pass to reuse the cfg_rt! macro inside the module (and add a similar cfg_metrics_rs_rt!. If only in the root, I can clean up the vestigial annotations inside the module.

[arielb1]

Approving if this is tested in docs.rs. If you figure out how to generate docs.rs like docs, could you put these instructions in a CONTRIBUTING.md file?

[jlizen]

@arielb1 added test instructions to CONTRIBUTING.md, it's pretty easy. I did validate that the annotations on the direct struct definitions are not needed if you annotate the re-exports from lib.rs only.

I'll go ahead and merge, but I'm not sure if we need manual action to stage this to rustdocs? Is our automation hooked into crates.io publish only?

[jlizen]

(From ariel offline - This will need to wait to go out until next crates.io version, probably not worth it just for this - i'll cut a tracking issue)