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.

[povilasv]

Shouldn't this be empty unit?

"1" is for utilization:

Instruments for utilization metrics (that measure the fraction out of a total) are dimensionless and SHOULD use the default unit 1 (the unity).

Ref: https://opentelemetry.io/docs/specs/semconv/general/metrics/#instrument-units

[ChrsMark]

I followed what

semantic-conventions/model/hardware/common-metrics.yaml

Lines 49 to 65 in a596f19

	- id: metric.hw.status
	type: metric
	metric_name: hw.status
	stability: experimental
	brief: "Operational status: `1` (true) or `0` (false) for each of the possible states"
	instrument: updowncounter
	unit: "1"
	extends: metric.hw.attributes
	note: >
	`hw.status` is currently specified as an *UpDownCounter* but would ideally be represented using a
	[*StateSet* as defined in OpenMetrics](https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#stateset).
	This semantic convention will be updated once *StateSet* is specified in OpenTelemetry. This planned change
	is not expected to have any consequence on the way users query their timeseries backend to retrieve the
	values of `hw.status` over time.
	attributes:
	- ref: hw.state
	requirement_level: required

defines 🤔 .

@open-telemetry/specs-semconv-maintainers any suggestion here?