[elastic-agent] fix: cpu cgroup values

[simitt]

What does this PR do?

Minor fix for collecting cgroup based metrics.

follow up of #22793

Why is it important?

cgroups metrics are not reported due to a typo

How to test

• create elastic agent docker image, e.g. DEV=true SNAPSHOT=true PLATFORMS=linux/amd64 TYPES=docker mage package
• run image and pass some quota, e.g. --cpu-period=100000 --cpu-quota=50000
• run load against APM Server, e.g. by using this testapp
• check metrics indexed to ES and ensure cgroup values are set

GET *metric*/_search
{
  "_source": ["system.process.cgroup"],
  "query": {
    "exists": {
      "field": "system.process.cgroup"
    }
  }
}

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[ruflin]

LGTM. Did not test it locally but definitively looks like a typo.

[simitt]

same fix as in #22879

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[elasticmachine]

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Build stats

• Build Cause: Pull request #23714 updated

• • Start Time: 2021-02-01T10:23:34.091+0000

• Duration: 90 min 32 sec

• Commit: c779607

Test stats 🧪

Test	Results
Failed	0
Passed	45151
Skipped	4737
Total	49888

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	45151
Skipped	4737
Total	49888

[simitt]

Failing E2E tests are unrelated.