Provide call that lifts http4s client into trace context without creating new span

[fiadliel]

There is a possible minimal implementation of tracing for a http4s client, which is to lift the context to one allowing tracing, and also forwards the tracing headers; but otherwise does not create a new span or decorate the current span.

Reasons?

• Firstly, it's a valid use case to not want the client to create a span. Maybe there are issues with forwarding trace data for this particular service, or the extra span adds unneeded extra information. However, you will still want to forward headers, so that downstream and upstream services can still be connected for the larger trace.
• Secondly, the creation of span attributes can be something that users want to customize. E.g. if you want to match OpenTelemetry attribute names, amongst other things. By putting the monad lifting into one place, it makes adding custom behavior much simpler, as you can just use Trace[F].span etc. inside your application code. It's basically a nice building block for custom client tracing schemes.

Example code - doesn't have a Ctx parameter, but shows the general idea:

  def liftTrace[F[_]: MonadCancelThrow, G[_]: MonadCancelThrow](
      client: Client[F],
      toHeaders: ToHeaders = ToHeaders.standard
  )(implicit P: Provide[F, G, Span[F]]): Client[G] =
    Client { (request: Request[G]) =>
      Resource.eval(P.ask).flatMap { span =>
        val traceHeaders = toHeaders
          .fromContext(span.context)
          .values
          .map(Header.Raw.apply)
          .toList

        client
          .run(
            request
              .transformHeaders(_ ++ Headers(traceHeaders))
              .mapK(P.provideK(span))
          )
          .mapK(P.liftK)
          .map(_.mapK(P.liftK))
      }
    }

[catostrophe]

IMO any outgoing call should have an explicit span. But I accept that users can decide for themselves.
PR is welcome.