[help wanted] Per-handler Micrometer tags on resicache.chain.execute timer

[DavidHLP]

Goal (v0.1.0, P0)

Make the chain timer queryable per handler + decision in Prometheus/Grafana.

Scope

• Add handler + decision tags to the existing resicache.chain.execute Timer in CacheHandlerChain (currently a single tag-less Timer).
• Cardinality bounded: handler (5) × decision (3) × cacheName (bounded). DO NOT add redisKey (unbounded, guide line 261).
• Note: R25 already added resicache.handler.fired counter (tag handler); this issue is the timer tags specifically.

Refs

• guide §223c / line 291 / line 261, wiki/modules/observability.md