The metadata cache reported (entry_count, 0) because moka exposes no hit
or miss counts, so the miss half of every cache report was a constant.
Track lookups and removals in the cache itself: hit/miss counters on the
lookup path, a moka eviction listener classifying removals by cause, and
an entry gauge refreshed whenever the entry set changes. The counters are
exported through the metrics facade under the rustfs_kms_ prefix with
static label values only, matching the operation-policy metrics, and are
also returned as a KmsCacheStats snapshot in place of the old tuple.
Cache semantics are unchanged: capacity, TTL and invalidation points are
the same, and remove now flushes pending maintenance so the gauge and the
removal notification describe the cache the caller sees.
Refs rustfs/backlog#1584
* refactor(sse): decouple encryption from ecstore
* feat(kms): enhance KMS service manager with runtime state and persistence support
* feat(kms): add local key export functionality for SSE-S3 migration tests
* fix(kms): keep local key export narrowly scoped
* fix(sse): validate copy source customer algorithm
---------
Co-authored-by: Zhengchao An <anzhengchao@gmail.com>
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