Fix bug with override path in cgroups

[fearful-symmetry]

What does this PR do?

This is a fix for #27598

In cases where cgroupsHierarchyOverride is set to /, and ignoreRootCgroups is set, we'll just completely bypass any cgroups reporting.

Why is it important?

It's a bug that was breaking containers.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

• Build metricbeat (or any other beat) for docker
• run on linux
• set http.enabled to true in the beat config
• From inside the docker container, do curl localhost:5066/stats?pretty
• Make sure cgroup metrics are reported.

Related issues

• Fixes libbeat/cmd/instance: cgroup metrics no longer being reported #27598

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2021-08-31T02:58:21.884+0000

• Duration: 148 min 56 sec

• Commit: 6f50223

Test stats 🧪

Test	Results
Failed	0
Passed	53721
Skipped	5325
Total	59046

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Test stats 🧪

Test	Results
Failed	0
Passed	53721
Skipped	5325
Total	59046

[axw]

Thanks @fearful-symmetry!

[matschaffer]

Is there a handy place I can snag the linux binary? I should be able to test it on my cloud deployment.

[ruflin]

@matschaffer Not before the merge of the PR. But I can build it for you or tell you how to build it ;-)

[fearful-symmetry]

/test

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b cgroupsHierarchyOverride-logic-fix upstream/cgroupsHierarchyOverride-logic-fix
git merge upstream/master
git push upstream cgroupsHierarchyOverride-logic-fix

[matschaffer]

@ruflin either way. Pretty sure you also have all the access you need to try to squeeze a build into a test deployment. Since @axw approved maybe we can just wait for the next BC.

[axw]

FWIW, I tested this with this small program - ran it before/after changes in a Docker container:

package main

import (
        "fmt"
        "log"

        "github.com/elastic/beats/v7/libbeat/cmd/instance/metrics"
        "github.com/elastic/beats/v7/libbeat/monitoring"
)

func main() {
        if err := metrics.SetupMetrics("testing"); err != nil {
                log.Fatal(err)
        }
        snapshot := monitoring.CollectFlatSnapshot(monitoring.Default, monitoring.Full, false)
        for name, value := range snapshot.Ints {
                fmt.Println(name, value)
        }
}

I think we're good, and can just wait for the next BC after this merges.

[fearful-symmetry]

Yep, sorry for the delay, once again fighting with CI...

[fearful-symmetry]

/test

[fearful-symmetry]

FINALLY