[receiver/k8scluster] Add `pods` field to `allocatable_types_to_report` parameter options

[odubajDT]

Component(s)

receiver/k8scluster

Describe the issue you're reporting

The receiver documentation is missing pods under the available options for allocatable_types_to_report, since this is available for nodes under k8s.node.allocatable_pods metric.

Field available in Node:

status:
  allocatable:
    cpu: "6"
    ephemeral-storage: 104266732Ki
    memory: 12215080Ki
    pods: "110"

[github-actions]

Pinging code owners:

• receiver/k8scluster: @dmitryax @TylerHelmuth @povilasv @ChrsMark

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