[CONTINT-5389] Collect kubelet_containers_per_pod_count in the kubelet check

[sehgal23]

What does this PR do?

• Documents the kubernetes.kubelet.containers_per_pod.count and .sum metrics in kubelet/metadata.csv, corresponding to the kubelet's kubelet_containers_per_pod_count histogram now collected by the Agent.

Motivation

https://datadoghq.atlassian.net/browse/CONTINT-5389

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• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
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[dd-octo-sts]

Validation Report

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Validation	Description	Status
agent-reqs	Verify check versions match the Agent requirements file	✅
ci	Validate CI configuration and code coverage settings	✅
codeowners	Validate every integration has a CODEOWNERS entry	✅
config	Validate default configuration files against spec.yaml	✅
dep	Verify dependency pins are consistent and Agent-compatible	✅
http	Validate integrations use the HTTP wrapper correctly	✅
imports	Validate check imports do not use deprecated modules	✅
integration-style	Validate check code style conventions	✅
jmx-metrics	Validate JMX metrics definition files and config	✅
labeler	Validate PR labeler config matches integration directories	✅
legacy-signature	Validate no integration uses the legacy Agent check signature	✅
license-headers	Validate Python files have proper license headers	✅
licenses	Validate third-party license attribution list	✅
metadata	Validate metadata.csv metric definitions	✅
models	Validate configuration data models match spec.yaml	✅
openmetrics	Validate OpenMetrics integrations disable the metric limit	✅
package	Validate Python package metadata and naming	✅
qa-label	Validate the pull request declares whether it needs QA for the next Agent release	✅
readmes	Validate README files have required sections	✅
saved-views	Validate saved view JSON file structure and fields	✅
version	Validate version consistency between package and changelog	✅

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[datadog-prod-us1-6]

 

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 91.09% (+2.77%)

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 1687a9bc9c

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[chatgpt-codex-connector]

Describe the cumulative histogram sum accurately

When the kubelet has observed pods over time, this Prometheus histogram's _sum is the cumulative sum of all container-count observations since the kubelet process started; it does not decrease when pods leave. Describing it as the total across all pods currently on the node will cause users to interpret or dashboard it as an instantaneous container count. Describe it as the cumulative number of containers across observed pod samples instead.

Useful? React with 👍 / 👎.

[justin-lesko]

We faced the same confusion so it's worth clarifying per Codex's suggestion.

I think you should also adopt the metric's official description.

The number of containers per pod.

For updated descriptions:

The cumulative sum of the number of containers per pod.

and

The count of the number of containers per pod.