[receiver/kubeletstats] Add feature gate for cpu utilization metrics' deprecation

[ChrsMark]

Description:

This PR adds a feature gate as discussed at #27885 (comment).
If the feature gate is enabled the container.cpu.utilization, k8s.pod.cpu.utilization and k8s.node.cpu.utilization metrics will be not be disabled being replaced by the container.cpu.usage, k8s.pod.cpu.usage and k8s.node.cpu.usage.

Feature gate schedule

• alpha: when enabled it makes the .cpu.usage metrics enabled by default
• beta: .cpu.usage metrics are enabled by default and any configuration enabling the deprecated .cpu.utilization metrics will be failing. Explicitly disabling the feature gate provides the old (deprecated) behavior.
• stable: .cpu.usage metrics are enabled by default and the deprecated metrics are completely removed.
• Removed three releases after stable.

@TylerHelmuth @dmitryax let me know what you think about this approach.

Link to tracking Issue:

Testing:

Documentation:

How to test this

1. Using the following configuration

mode: daemonset
presets:
  kubeletMetrics:
    enabled: true

image:
  repository: otelcontribcol-dev
  tag: "latest"
  pullPolicy: IfNotPresent

command:
  name: otelcontribcol
  extraArgs: [--feature-gates=receiver.kubeletstats.enableCPUUsageMetrics]

config:
  exporters:
    debug:
      verbosity: normal
  receivers:
    kubeletstats:
      collection_interval: 10s
      auth_type: 'serviceAccount'
      endpoint: '${env:K8S_NODE_NAME}:10250'
      insecure_skip_verify: true

  service:
    pipelines:
      metrics:
        receivers: [kubeletstats]
        processors: [batch]
        exporters: [debug]

2. Ensure that only the .cpu.usage metrics are reported.
3. Disable the feature gate and check that only the .cpu.utilization metrics are reported.

[povilasv]

I would maybe add a note that usage metrics are not utilization metrics. So it's not one to one replacement.

Something like:

Note: usage metrics show usage in terms of resources such as CPU / bytes, not percentage used.
Previous utilization metric values were computed incorrectly.

[povilasv]

Thanks!