Integrate LabKit for Distributed Tracing

Rails, Sidekiq, Gitaly and Workhorse are now instrumented with distributed tracing. Gitaly and Workhorse use LabKit, a go library, for this.

We should add distributed tracing support to gitlab-runner too.

Repository is at: https://gitlab.com/gitlab-org/labkit

Documentation is at: https://godoc.org/gitlab.com/gitlab-org/labkit/tracing

I haven't looked at the runner code yet, but this might be as easy as add the tracing Roundtripper to any HTTP clients used to make requests from the runner to the main application.

cc @dosuken123