Support Opentracing

Currently graphql-go/graphql is the only one of the major graphql libs for Go that doesn't support [opentracing](https://opentracing.io/guides/golang/). Ideally an opentracing implementation would create spans for each query and mutation and tag each trace with the request string.

My organization is trying to instrument APM/Distributed tracing on all of our new applications and ideally we'd like to have the ability to trace each query/mutation submitted against our app which would help us monitor our GQL app at scale.