support for istio distributed tracing

Hi guys,

I've been using go-kit for some times now (high five for the amazing work !) and have been developing micro-services on Kubernetes and istio where istio tracing is leveraged. If interested in supporting istio tracing through simple header propagation ([istio doc](https://istio.io/faq/distributed-tracing/#how-to-support-tracing)), I have working code that I would be happy to push through a pull request.

Basically the code implements the function `func ServerBefore(before ...RequestFunc) ServerOption` for HTTP and gRPC

defined here
* https://github.com/go-kit/kit/blob/master/transport/http/server.go#L51
* https://github.com/go-kit/kit/blob/master/transport/grpc/server.go#L60

so that header values can be saved in the `context`, and then inject to every downstream requests.

Is it something you see being pushed to your code base ? Let me know.
Cheers