Cg2: Add the ability to filter stats

[dcantah]

A large user of this library (unsurprisingly) is the containerd daemon, and specifically through the CRI entrypoint. There's quite a few stats we gather that are never bubbled up the chain when requested through that funnel. The stats never used are:

MemoryEvents, Rdma, Hugetlb, and IO.

We should likely do the same to cg1 (#370), but if we want to add it to the Cgroup interface it'd be a breaking change.

cpu: AMD Ryzen 9 9950X3D 16-Core Processor

BenchmarkStatFiltered/StatAll-32                    5547            619802 ns/op           48778 B/op        316 allocs/op
BenchmarkStatFiltered/CPUOnly-32                   59121             60774 ns/op           10032 B/op         43 allocs/op
BenchmarkStatFiltered/MemoryOnly-32                16192            222286 ns/op           15552 B/op        158 allocs/op
BenchmarkStatFiltered/CPUAndMemory-32              12670            286484 ns/op           25440 B/op        200 allocs/op