cgroups: Adjust metric names & disable metrics with config

cgroups: Only enable metrics if option is turned on

On many cloud providers, cadvisor runs and keeps track of cgroups. To have Gitaly expose its own metrics is redundant at this point. Allow users to configure whether or not they would like Gitaly to collect its own metrics about cgroups.

Changelog: added

cgroups: Adjust metric names

Change the metric names to be more accurate. The memory.failcnt tracks how often the kernel has tried to reclaim memory. While this maps closely to the number of killed processes, they are not exactly the same thing.

The cgroups cpu usage metrics are a running total. Add "total" at the end of the metric to indicate this.

Changelog: changed