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

[janiussyafiq]

Description

Add a new per-metric configuration option to the Prometheus plugin's plugin_attr:

• disable_labels: A list of built-in label names whose values will be collapsed to an empty string "" for a given metric, reducing cardinality without changing the metric schema.

This is a non-breaking change — all labels remain registered and present in the output, so existing Prometheus dashboards and recording rules are unaffected. Only the label values of the specified labels are zeroed out.

An alternative approach of disable: true (removing a metric entirely from /metrics) was considered but excluded from this PR as it is a breaking change — it would cause dashboards to show "No data", misfire absent() alerts, and break recording rules. Instead, the same effect of suppressing all label cardinality on a metric can be achieved by listing all its labels under disable_labels, which keeps the metric present in the output with empty-string values while remaining fully non-breaking.

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
      http_latency:
        disable_labels:
          - node

With the above config, node and consumer will appear as node="",consumer="" in the metric output instead of carrying real values, effectively collapsing all time series that differ only by those labels.

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)