LabKit integration with Gitaly-Ruby

This change adds distributed tracing instrumentation to Gitaly-Ruby, so that the process can emit distributed tracing spans to Jaeger.

Depends on

• https://gitlab.com/gitlab-org/labkit-ruby/merge_requests/1
• https://gitlab.com/gitlab-org/labkit-ruby/merge_requests/4

Related:

• GitLab-CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25379

• GitLab-EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9605

What does this give us?

Gitaly-Ruby Server Spans

Calls from Gitaly to Gitaly-Ruby will be instrumented on both sides (client = Gitaly and server = Gitaly-Ruby).

Gitaly-Ruby forms part of the Jaeger server dependency DAG