Add cgroup observer to prevent infinite fanotify growth

[bobrik]

With fanotify we see every cgroup appear. Even if there are no metrics associated with it and the cgroup quickly goes away, we keep the mapping in memory. This mostly works fine, until you have an infinite churn of cgroups, then memory starts growing indefinitely.

This commit adds a garbage collecting cache that would remove old entries that were not requested in a long time.

[bobrik]

Heap profile from a problematic instance:

Looking with a debugger at another instance:

$ sudo dlv attach $(pidof prometheus-ebpf-exporter)
(dlv) b github.com/cloudflare/ebpf_exporter/v2/cgroup.(*fanotifyMonitor).Resolve
Breakpoint 1 set at 0x851b6e for github.com/cloudflare/ebpf_exporter/v2/cgroup.(*fanotifyMonitor).Resolve() /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/cgroup/fanotify.go:185
(dlv) c
(dlv) bt
 0  0x0000000000851b6e in github.com/cloudflare/ebpf_exporter/v2/cgroup.(*fanotifyMonitor).Resolve
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/cgroup/fanotify.go:185
 1  0x000000000085c899 in github.com/cloudflare/ebpf_exporter/v2/cgroup.(*Monitor).Resolve
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/cgroup/monitor.go:35
 2  0x000000000085c899 in github.com/cloudflare/ebpf_exporter/v2/decoder.(*CGroup).Decode
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/decoder/cgroup.go:33
 3  0x000000000085d1e7 in github.com/cloudflare/ebpf_exporter/v2/decoder.(*Set).decode
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/decoder/decoder.go:75
 4  0x000000000085ddde in github.com/cloudflare/ebpf_exporter/v2/decoder.(*Set).decodeLabels
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/decoder/decoder.go:155
 5  0x000000000085d5ca in github.com/cloudflare/ebpf_exporter/v2/decoder.(*Set).DecodeLabelsForMetrics
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/decoder/decoder.go:108
 6  0x0000000000aa18ca in github.com/cloudflare/ebpf_exporter/v2/exporter.(*Exporter).mapValues
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/exporter/exporter.go:581
 7  0x0000000000aa043c in github.com/cloudflare/ebpf_exporter/v2/exporter.(*Exporter).collectCounters
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/exporter/exporter.go:467
 8  0x0000000000a9fb3e in github.com/cloudflare/ebpf_exporter/v2/exporter.(*Exporter).Collect
    at /cfsetup_build/prometheus-ebpf-exporter/tmp/build/cloudflare-ebpf_exporter-9a207bb/exporter/exporter.go:455
 9  0x0000000000921c65 in github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func1
    at /home/builder/go/pkg/mod/github.com/prometheus/client_golang@v1.19.1/prometheus/registry.go:455
10  0x000000000047a781 in runtime.goexit
    at /usr/local/go/src/runtime/asm_amd64.s:1695
(dlv) print len(m.mapping)
2276851

There are definitely fewer than 2 million cgroups on this instance:

$ find /sys/fs/cgroup -type d | wc -l
665