Support cgroup v2 CPU stats when controller not enabled

[jay-mckay]

manager.Stat() iterates over enabled controllers to construct statistics, namely for memory and CPU. If a controller is not enabled, the statistics are not provided.

cgroups/cgroup2/manager.go

Lines 565 to 575 in 0c03de4

	for _, controller := range controllers {
	switch controller {
	case "cpu", "memory":
	if err := readKVStatsFile(c.path, controller+".stat", out); err != nil {
	if os.IsNotExist(err) {
	continue
	}
	return nil, err
	}
	}
	}

However, regardless of whether the cpu controller is enabled, cgroups v2 always provides the following statistics in cpu.stat :

• usage_usec
• user_usec
• system_usec

Systemd no longer enables this controller when the property CPUAccounting=True, meaning these statistics are not provided when they probably should be. Stat() should be modified to fix this.

See https://docs.kernel.org/admin-guide/cgroup-v2.html#cpu-interface-files, systemd/systemd#10507

[jay-mckay]

I believe this straightforward. If this behavior is desired, I can work on the fix.