Create mtail sidecar container to gather metrics from gitlab-shell

Currently our VM's utilize the mtail service that is not currently shipped with omnibus nor our helm chart to gather metrics from the GitLab.com infrastructure. Utilize this issue to build a sidecar configuration complete with appropriate configuration to advertise metrics that can be utilize to determine the health of this service.

Closing Criterion

• Build a sidecar container
• Build an appropriate mtail configuration
• Deploy it
• Configure prometheus to scrape it
• Build/Bolster existing dashboards
• Create/Modified alerts as necessary