Server middleware support

[lacarvalho91]

Overview

I'd like to be able to configure a server with a middleware that would run on each request, similar to what is possible with http4s middleware.

Where this idea has come from is I've been writing servers that use otel4s to attach to existing traces (passed to the server as metadata). This works fine, but the problem is this needs to be done for every server implementation. I would instead like to be able to do this in a generic way and be able to configure the server with this behaviour.

I can't really use gRPC interceptors for this, because the state of the context in cats.mtl.Local (which otel4s and other libs use) is scoped only to the effect passed into local.

From looking into the code, we could intercept the request in the Fs2ServerCallHandler and wrap the implementation: (Request, Metadata) => F[Response] in a middleware. Then allow passing middlewares in the GeneratedCompanion.service.

[lacarvalho91]

Does this seem achievable?

[ahjohannessen]

I do not have many spare cycles these days, but you are welcome to give it a go with a PR and I will take a look :)

[iRevive]

I think this one can be closed since #669 is merged and released.