Support open-tracing

[obukhov]

Gateway works with multiple underlying (implementing) services. To analyze its performance it would be nice to support OpenTracing (https://medium.com/opentracing/towards-turnkey-distributed-tracing-5f4297d1736) this actually can be achieved through middleware but can be tricky sometimes.

I think supporting open-tracing standard to unify distributed tracing will be beneficial for most uses. What do you think?
If you like the idea I can work on it as a hacktoberfest :)

[AlecAivazis]

I would love to have some kind of support for analyzing the performance of the gateway at the resolver level. If we can find a way to integrate this into standards adopted by larger community, that's even better! Something to consider if if there is benefit supporting the same format of tracing that apollo's platform supports (described here).

Please let me know if you need any kind of guidance here. This has been a long running "next step" for me that I haven't found the time to dedicate myself to. I'm usually available on the gopher slack if you want to talk through anything.