Add comprehensive cache statistics

The `CacheStats` class provides detailed metrics like hit/miss counts,
load success/failure counts, total load time, and eviction counts.
It also offers derived metrics such as hit/miss rates, load failure rate,
and average load penalty. The design is inspired by Caffeine.

`BasicClientSideCache` now uses a `StatsCounter` to accumulate these
statistics, exposed via a new `stats()` method. The previous
`cacheHits()` and `cacheMisses()` methods have been removed.

A `recordStats` option (default: true) in `ClientSideCacheConfig`
allows disabling statistics collection.

Author: bobymicroby

docs/v5.md

@@ -104,7 +104,7 @@ There are two ways to implement client side caching:
 
 ```javascript
 const client = createClient({
-  RESP: 3, 
+  RESP: 3,
   clientSideCache: {
     ttl: 0,             // Time-to-live in milliseconds (0 = no expiration)
     maxEntries: 0,      // Maximum entries to store (0 = unlimited)
@@ -142,8 +142,17 @@ cache.invalidate(key);
 cache.clear();
 
 // Get cache metrics
-const hits = cache.cacheHits();
-const misses = cache.cacheMisses();
+// `cache.stats()` returns a `CacheStats` object with comprehensive statistics.
+const statistics = cache.stats();
+
+// Key metrics:
+const hits = statistics.hitCount;        // Number of cache hits
+const misses = statistics.missCount;      // Number of cache misses
+const hitRate = statistics.hitRate();     // Cache hit rate (0.0 to 1.0)
+
+// Many other metrics are available on the `statistics` object, e.g.:
+// statistics.missRate(), statistics.loadSuccessCount,
+// statistics.averageLoadPenalty(), statistics.requestCount()
 ```
 
 ## Pooled Caching