CFP-26521: Expose URL path in Hubble HTTP metrics

[amaanx86]

tl;dr: add an opt-in path label to the Hubble HTTP Prometheus metrics, with operator-supplied templates that collapse variable URL segments (IDs, slugs) into stable placeholders, and a hard cardinality cap.

The original issue was closed in 2023 on cardinality grounds and reopened in 2024 once the discussion moved toward a templated, opt-in design. This CFP writes that design down so implementation work can start. Key points:

• New optional pathContext block in the HTTP metric config; fits both the legacy hubble-metrics config string and the dynamic config map from CFP-30788.
• Templates are operator-supplied, matched in order, with a maxCardinality ceiling and a configurable miss policy (drop_label, drop_sample, bucket_other).
• Two small self-observability metrics for the matcher itself.
• Backwards compatible: if pathContext is unset, the metric labels are identical to today.

Pulling templates from L7 CiliumNetworkPolicy rules (suggested by @apt-itude on the issue) is discussed as an alternative configuration source and deferred to a follow-up CFP.

Cilium issue: cilium/cilium#26521