This change is a fairly straightforward refactor to extract the tracing
and correlation-id code from the gitlab rails codebase into the new
LabKit-Ruby project.

The corresponding import into LabKit-Ruby was in
https://gitlab.com/gitlab-org/labkit-ruby/merge_requests/1

The code itself remains very similar for now.

Extracting it allows us to reuse it in other projects, such as
Gitaly-Ruby. This will give us the advantages of correlation-ids and
distributed tracing in that project too.

Jaeger is a distributed tracing tool. This change adds a "Tracing" link
to the performance bar to directly link to a current request in Jaeger.

This is useful for two reasons: 1 - it provides affordance to developers
that the distributed tracing tool is available, so that it can quickly
be discovered. 2 - it allows developers to quickly find a specific trace
without having to manually navigate to a second user-interface.