Support RequestSampler in zipkin server middleware

[jcchavezs]

zipkin-go includes an option missing in this server middleware, RequestSampler which allows to change the sampling decision based on the request. This is specially useful when you want to sample requests coming from async operations (that often don't include tracing propagation) and in general to give more granularity to the sampling.

We are lacking that option in this server middleware, should I open a PR for that?

Ping @basvanbeek

[basvanbeek]

Yes that would be a good addition, especially for frontend services.