Re-introduce per-process CPU collection (#1146)

### What does this PR do?

This commit re-introduces per-process CPU data from /proc/{pid}/stat reusing the
same naming scheme from the cgroup sourced data, offset with stat. to avoid
summing issues in aggregations.

I have removed much of the stat code from the main procfs sample loop. The data
was either no longer used or is duplicated elsewhere. Some data could be
re-introduced if we desire by extending the poll loop in stat.rs.

I am continuing to remove more and more of our procfs crate integration. I
think, ultimately, we should be able to parse directly without the need of a
third-party dependency. That removal is near done now.

lading/src/observer/linux/cgroup.rs

@@ -1,5 +1,5 @@
 /// Code to read cgroup information.
-mod v2;
+pub(crate) mod v2;
 
 use std::{collections::VecDeque, io};
 

lading/src/observer/linux/cgroup/v2/cpu.rs

@@ -96,6 +96,9 @@ pub(crate) async fn poll(group_prefix: &Path, labels: &[(String, String)]) -> Re
         let user_fraction = delta_user as f64 / delta_time;
         let system_fraction = delta_system as f64 / delta_time;
 
+        // NOTE these metric names are paired with names in procfs/stat.rs and
+        // must remain consistent. If you change these, change those.
+
         // Convert usage to a percentage of the cores granted to the target.
         let total_cpu = (usage_fraction / allowed_cores) * 100.0;
         let user_cpu = (user_fraction / allowed_cores) * 100.0;
@@ -106,7 +109,7 @@ pub(crate) async fn poll(group_prefix: &Path, labels: &[(String, String)]) -> Re
         gauge!("kernel_cpu_percentage", labels).set(system_cpu); // kernel is a misnomer, keeping for compatibility
         gauge!("system_cpu_percentage", labels).set(system_cpu);
 
-        // Convert usage to kubernetes style millicores. These
+        // Convert usage to kubernetes style millicores.
         let total_millicores = usage_fraction * 1000.0;
         let user_millicores = user_fraction * 1000.0;
         let system_millicores = system_fraction * 1000.0;