[k8s] Add k8s node allocatable metrics

[odubajDT]

Fixes #2243

Changes

added

k8s.node.allocatable_cpu
k8s.node.allocatable_ephemeral_storage
k8s.node.allocatable_memory
k8s.node.allocatable_pods

metrics which are already used in k8scluster receiver

Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [x] Change log entry added, according to the guidelines in When to add a changelog entry.