cgroups V2 metrics should report limits for the stats HTTP endpoint

[fearful-symmetry]

Filed with the help of @miltonhultgren and @axw

Right now, cgroups V1 metrics will report limit information for the memory controller. Under C2, none of the controllers report limits for cgroups, only metrics. If we want this info to appear in the /stats endpoint used by the beats, two things need to happen:

• we need to update the cgv2 reporters to fetch the limit values, usually in .max and .high files, for the controllers
• update the reporters in report.go, that are used to fetch data for the HTTP /stats endpoint, to actually report those values

[simitt]

To add some context - this issue was identified as the root cause for a bug in APM Server stack monitoring elastic/apm-server#8596.