ENG-3727 Observability: measure how long are data kept in cache

PR Summary

Low Risk
Low risk: adds metrics instrumentation and a small in-memory timestamp map, without changing cache eviction or diff cleanup behavior. Main concern is minor additional memory/CPU overhead and potential skew if items are overwritten without updating the original insert time.

Overview
Adds OpenTelemetry instrumentation to record a histogram (orchestrator.build.cache.residence_duration) for how long a cached build diff remains in the local build cache, tracking insertion time per diff key and recording the duration on cache eviction.

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