[prometheus-blackbox-exporter] use kubernetes.io/metadata.name in networkPolicy

[firasmosbehi]

Summary

This PR updates prometheus-blackbox-exporter network policy namespace matching to use Kubernetes' well-known namespace label and addresses #6640.

Problem

When networkPolicy.allowMonitoringNamespace is enabled, the chart currently matches on name=<namespace>. In many environments, deployers do not control namespace labels and cannot guarantee a custom name label exists.

Root cause

templates/networkpolicy.yaml hardcodes the namespace selector key to name.

Fix

• Switched namespace selector label key from name to kubernetes.io/metadata.name.
• Updated values comment to describe the new expected label semantics.
• Bumped chart version from 11.8.0 to 11.8.1.

Validation

• helm lint charts/prometheus-blackbox-exporter
• helm template test charts/prometheus-blackbox-exporter --set networkPolicy.enabled=true --set networkPolicy.allowMonitoringNamespace=true --set networkPolicy.monitoringNamespaceName=observability
• Verified rendered selector contains kubernetes.io/metadata.name: observability.

Notes

This keeps the same user-facing value (monitoringNamespaceName) while aligning selector behavior with a standard Kubernetes label.

[monotek]

As this is changing the current default behaviour i consider this a breaking change.
Therefore major version of the chart woul dneed an update.
Maybe make key and value configurable so the current default can be kept.