Add k8s.namespace.phase metric and attribute

[ChrsMark]

Part of #1032

Changes

This PR adds the k8s.namespace.phase as metric along with its respective attribute k8s.namespace.phase.

This metric is already in use by the Collector and specifically the k8scluster receiver.

The introduced attribute is new in order to follow the modeling that is already in use by hw metrics and jmx metrics mentioned at #1032 (comment) (see #1554).

Collector implementation: https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/9dfa2f7813b11500d001622f3d8c1dd8d9ac58fd/receiver/k8sclusterreceiver/internal/namespace/namespaces.go#L14

K8s API ref: https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.30/#namespacestatus-v1-core

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[github-actions]

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[trask]

is "phase" a common term in the context of k8s namespaces? (i.e. vs status or state)

[ChrsMark]

is "phase" a common term in the context of k8s namespaces? (i.e. vs status or state)

I believe so. It's what the API defines and I see it's used in the docs description as well:

A namespace can be in one of two phases:

- Active the namespace is in use
- Terminating the namespace is being deleted, and can not be used for new objects