Fix process metrics collection when both cgroup V1 and V2 controllers exist

What does this PR do?

This PR fixes a bug in the cgroup module of process metrics collection when both cgroup V1 and V2 controllers exist but /sys/fs/cgroups/unified is not mounted.

This case can exist on Kubernetes nodes where systemd is configured to use the cgroup V1 hierarchy but a service needs to mount the cgroup V2 hierarchy for its own use (This is what Cilium does for example). In that case, the proc tree contains a reference to a empty cgroup V2 controller which confuses metricbeat which report a PID XXX contains a cgroups V2 path (0::/) but no V2 mountpoint was found.\nThis may be because metricbeat is running inside a container on a hybrid system.\nTo monitor cgroups V2 processess in this way, mount the unified (V2) hierarchy inside\nthe container as /sys/fs/cgroup/unified and start the system module with the hostfs setting. error.

Why is it important?

This fixes gathering process metrics on Amazon Kubernetes nodes running Cilium.

Checklist

[x] My code follows the style guidelines of this project
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have added an entry in CHANGELOG.md

Author's Checklist

This PR is split in 2 commits:

a test case to reproduce the issue with the existing code
a logic change in to fix the issue

Running tests with only the first commit leads to this error:

?       github.com/elastic/elastic-agent-system-metrics/metric/system/cpu   [no test files]
--- FAIL: TestReaderGetStatsV1MalformedHybrid (0.00s)
    reader_test.go:92: 
            Error Trace:    reader_test.go:92
            Error:          Received unexpected error:
                            error fetching cgroupV2 controllers for cgroup location '' and path line '0::/': open testdata/amzn2/sys/fs/cgroup/unified: no such file or directory
            Test:           TestReaderGetStatsV1MalformedHybrid
            Messages:       error in GetV1StatsForProcess
--- FAIL: TestProcessCgroupHybridPaths (0.00s)
    util_test.go:212: error in ProcessCgroupPaths: error fetching cgroupV2 controllers for cgroup location '' and path line '0::/': open testdata/amzn2/sys/fs/cgroup/unified: no such file or directory
FAIL

elastic / elastic-agent-system-metrics