Support Opentracing

[awiede]

Currently graphql-go/graphql is the only one of the major graphql libs for Go that doesn't support opentracing. 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.