Context service

[fedefernandez]

This PR makes the Span[F] context abstract so that you can make your service dependant on any "context."

The idea comes from #1328

Pending tasks:

• Add tests for a custom context
• Update docs -> Will do in the following PR

Currently, we provide some methods in the companion object for getting a parameterized algebra over Span[F] with Kleisli

Given:

trait MyService[F]

The method MyService.tracingClient[F[_]] provides a:

Resource[F, MyService[Kleisli[F, Span[F], *]]]

And the method MyService.bindTracingService[F[_]] asks for an implicit instance implementation of the algebra parametized with Kleisli[F, Span[F], *] (MyService[Kleisli[F, Span[F], *]]).

In this PR, we're adding alternative methods to those "tracing" ones where you can specify any type as a context as soon you provide a couple of implicit implementations.

The new method MyService.contextClient[F[_], C] will require an implicit ClientContext[F, C]:

mu-scala/modules/service/src/main/scala/higherkindness/mu/rpc/internal/context/ClientContext.scala

Lines 6 to 15 in 9a0e04b

	final case class ClientContextMetaData[C](context: C, metadata: Metadata)
	trait ClientContext[F[_], C] {
	def apply[Req, Res](
	descriptor: MethodDescriptor[Req, Res],
	channel: Channel,
	options: CallOptions,
	current: C
	): Resource[F, ClientContextMetaData[C]]

While the new method MyService.bindContextService[F[_]] will require an implicit ServerContext[F, C]

mu-scala/modules/service/src/main/scala/higherkindness/mu/rpc/internal/context/ServerContext.scala

Lines 6 to 10 in 9a0e04b

	trait ServerContext[F[_], C] {
	def apply[Req, Res](descriptor: MethodDescriptor[Req, Res], metadata: Metadata): Resource[F, C]
	}

I've chosen traits because the instances need to be parameterized over F and C but allow any Req and Res (this is passed in the apply). I'm open to suggestions for a more ergonomic or better way to provide these implementations.

[cb372]

LGTM. It's probably worth updating the docs (e.g. https://higherkindness.io/mu-scala/guides/distributed-tracing) and the examples repo to reflect these changes. And maybe add a small how-to guide to the docs, showing how to use this feature to access the metadata?

[fedefernandez]

@cb372 yes! Initially, I was thinking about adding that in this PR, but this is getting bigger, and I prefer to keep this for code/implementation discussions and then open an issue to address that. Good point with the guide

[juanpedromoreno]

Besides my minor comment, shall we tag this PR as breaking-change?

[fedefernandez]

@juanpedromoreno this should be backward compatible (besides the deprecated warning) so I think this should be fine without the breaking-change tag. I'll take a look at the val names.