GitLab Shell should utilise LabKit observability functionality

LabKit, https://gitlab.com/gitlab-org/labkit, is a platform library for GitLab's Go services providing Logging, Monitoring, Tracing and CorrelationID services.

It is used by Workhorse, Gitaly, Pages and other services.

GitLab-Shell should adopt LabKit for:

1. Distributed Tracing ... done
2. CorrelationIDs ... done
3. Logging ... !504 (merged)