Add command to cgroups with feature flag

In order to roll out cgroups in a safe way, we need to control how commands are added to the cgroup with a feature flag.

This will allow us to slowly ramp up traffic and monitor how often a process gets killed.

This change also adds a metric to track how many commands have been killed. In order to gain visibility into the overall health and behavior of the gitaly fleet, it would be very useful to know how many git processes and which ones are being killed by an OOM killer. This is doubly the case if we are adding commands to cgroups and applying limits on memoory. This will give us visibility so we can adjust the memory limit in the cgroups config.

Post Merge Steps

• explicitly set feature flag to false in staging
• explicitly set feature flag to false in prod