Implements per-collection request telemetry with an LRU-bounded storage.

• Tracks requests per collection (Actix + Tonic collectors)
• Adds composite key for collection+endpoint
• Keeps memory bounded via LRU eviction
• Includes per-collection stats in requests telemetry output
• Does NOT modify LFS test artifacts (storage.tar.xz)

Note: This is a clean re-PR rebuilt via cherry-picks on top of current master after the previous branch got into a bad state due to GitHub UI conflict resolution affecting an LFS-tracked file. This PR introduces the per-collection request telemetry data layer (LRU-bounded) and wires it into request telemetry output. Exposing these metrics via Prometheus per-collection labels is intended as a follow-up step for #3322. /claim #3322 fixes #3322