Reduce CPU consumption and heap allocations of SubsystemMountpoints()

[rockdaboot]

What does this PR do?

This PR reduces memory allocations and thus GC activity in a hot path (SubsystemMountpoints()).

It also adds a benchmark that can be run with

cd metric/system/cgroup
go test . -bench SubsystemMountpoints -run nothing -count=1

Benchmark results Before

goos: linux
goarch: amd64
pkg: github.com/elastic/elastic-agent-system-metrics/metric/system/cgroup
cpu: 12th Gen Intel(R) Core(TM) i7-12800H
BenchmarkSubsystemMountpoints-20           24133             47672 ns/op           17176 B/op        159 allocs/op

Benchmark results After

goos: linux
goarch: amd64
pkg: github.com/elastic/elastic-agent-system-metrics/metric/system/cgroup
cpu: 12th Gen Intel(R) Core(TM) i7-12800H
BenchmarkSubsystemMountpoints-20           38503             33503 ns/op            7000 B/op         39 allocs/op

It became aware when looking at the Universal Profiling flamegraph for auditbeat that
SubsystemMountpoints() was consuming ~25% of the CPU usage of auditbeat. Additionally, it created a lot of unneeded heap allocations for temporary (function-local) variables. The GC ate another ~21% of the total CPU consumption of auditbeat.

Screenshot (data from a real set of >200 Elasticsearch clusters)

Why is it important?

As long running applications, the beats should consume as less CPU as possible.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works