Opt-in tracing support

[chris-ramon]

Overview

• closes: Support Opentracing #426
• This is a WIP support for Trace interface, so opentracing can be integrated.
• This PR will only adds opt-in Tracing support, and opentracing support implementation itself will live outside graphql-go/graphql so we don't have opentracing as dependency.
• Refer to a full working example to the new repo: opentracing-example:
  

Test plan

• Unit tests.

[Fontinalis]

@chris-ramon When will this land in the master branch? Also, any help needed with this one? Happy to help

[coveralls]

Coverage increased (+0.04%) to 91.912% when pulling acd9fcd on tracing-support into 21fe71d on master.

[chris-ramon]

@chris-ramon When will this land in the master branch? Also, any help needed with this one? Happy to help

Thanks a lot for the pre-disposition to help @Fontinalis. 👍

Once this PR lands master, one great task as a help-out to this PR would be documentation on how to enable tracing, perhaps a simple example using a tracing provider from your best interest.

[linniksa]

Can you look at https://github.com/apollographql/apollo-tracing extension , it more useful in dev environment (https://github.com/prisma/graphql-playground supports it)
maybe allow trace parse/validate stage?
is current Tracer interface allow return "path" field value as described in apollo-tracing extension (full path from root like in errors)?

[Fontinalis]

@chris-ramon I created a PR #448 that would add support for extensions.
@linniksa In my PR, there's an example (`star-wars-with-trace') that has a simple tracer, that basically adds the support for the "apollo-tracing". The field's resolve info can be viewed from an extension, so the full "path" field could be viewed.