Document an example of tracing end-to-end

The tracing capability looks very good and promising. I wonder if you have a snippet how would you recommend to integrate it with Zipkin or OpenTracing?

Also, the `TraceBuilder` type signature is `TraceBuilder = String => HttpClient => HttpClient`, which makes it more difficult to use it for tracing something that is not represented as HTTP, for instance, a database query. What would be the recommended way of doing it?

I am happy to contribute to the docs about tracing. So far the test case for full tracing was the main inspiration, but it would be great to find out how do people use it in production.

Thank you!