What
---
- Add a new configuration under `cgroups` called `cpu_quota_us` to
  configure `cfs_quota_us` for the parent cgroup
  https://docs.kernel.org/scheduler/sched-bwc.html?highlight=cfs_quota_us
- Add a new configuration under `cgroups.repositories` called
  `cpu_quota_us` to configure `cfs_quota_us` for the repository cgroup
  https://docs.kernel.org/scheduler/sched-bwc.html?highlight=cfs_quota_us
- Add metrics
    - `gitaly_cgroup_cpu_cfs_periods_total`: Read from `cpu.stat` nr_periods https://docs.kernel.org/scheduler/sched-bwc.html#statistics
    - `gitaly_cgroup_cpu_cfs_throttled_periods_total`: Read from `cpu.stat` nr_throttled https://docs.kernel.org/scheduler/sched-bwc.html#statistics
    - `gitaly_cgroup_cpu_cfs_throttled_seconds_total`: Read from `cpu.stat` throttled_time https://docs.kernel.org/scheduler/sched-bwc.html#statistics
- Add more test coverage when only specific values are set.

Why
---
At the moment we limit memory and CPU via
[`cpu.shares`](https...