feat(prometheus): support disabling labels and metrics to reduce cardinality

[janiussyafiq]

Description

Add two new per-metric configuration options to the Prometheus plugin's plugin_attr:

• disable_labels: A list of built-in label names to drop from a metric, reducing cardinality without changing the metric schema for other labels.
• disable: A boolean flag to skip registering an entire metric, removing it from /metrics output entirely.

Both options are configured under plugin_attr.prometheus.metrics.<metric_name> alongside the existing expire and extra_labels fields.

Example config:

  plugin_attr:                                                                                                                           
    prometheus:   
      metrics:
        http_status:
          disable_labels:
            - node
            - consumer                                                                                                                   
        bandwidth:
          disable: true

This addresses high-cardinality issues in dynamic environments (e.g. Kubernetes autoscaling where pod IPs churn rapidly), which can cause prometheus shared dict overflow and excessive memory consumption.

Which issue(s) this PR fixes:

Fixes #12679

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)