feat(kms): record real cache hit, miss and eviction metrics

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

crates/kms/examples/kms_local_demo.rs

@@ -227,10 +227,12 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     // Step 12: Check cache statistics
     println!("12. Checking cache statistics...");
-    if let Some((hits, misses)) = encryption_service.cache_stats().await {
+    if let Some(stats) = encryption_service.cache_stats().await {
         println!("   ✓ Cache statistics:");
-        println!("     - Cache hits: {}", hits);
-        println!("     - Cache misses: {}\n", misses);
+        println!("     - Cached entries: {}", stats.entries);
+        println!("     - Cache hits: {}", stats.hits);
+        println!("     - Cache misses: {}", stats.misses);
+        println!("     - Entries evicted: {}\n", stats.evictions);
     } else {
         println!("   - Cache is disabled\n");
     }

crates/kms/examples/kms_vault_kv_demo.rs

@@ -263,10 +263,12 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     // Step 12: Check cache statistics
     println!("12. Checking cache statistics...");
-    if let Some((hits, misses)) = encryption_service.cache_stats().await {
+    if let Some(stats) = encryption_service.cache_stats().await {
         println!("   ✓ Cache statistics:");
-        println!("     - Cache hits: {}", hits);
-        println!("     - Cache misses: {}\n", misses);
+        println!("     - Cached entries: {}", stats.entries);
+        println!("     - Cache hits: {}", stats.hits);
+        println!("     - Cache misses: {}", stats.misses);
+        println!("     - Entries evicted: {}\n", stats.evictions);
     } else {
         println!("   - Cache is disabled\n");
     }