introduce new stats item EvictedBytes

 It's incremented on bucket clean up and reports bytes
evicted from memory. It should help to understand an amount of extra
memory needed for fastcache to serve requests rate on constantly evicted
keys.

Related PR VictoriaMetrics/VictoriaMetrics#9293

Author: f41gh7

fastcache.go

@@ -237,8 +237,9 @@ type bucket struct {
 	// idx points to chunks for writing the next (k, v) pair.
 	idx uint64
 
-	collisions  uint64
-	corruptions uint64
+	collisions   uint64
+	corruptions  uint64
+	evictedBytes uint64
 }
 
 func (b *bucket) Init(maxBytes uint64) {
@@ -376,6 +377,7 @@ func (b *bucket) Set(k, v []byte, h uint64) {
 	b.idx = idxNew
 	if needClean {
 		b.cleanLocked()
+		atomic.AddUint64(&b.evictedBytes, uint64(len(chunks)*chunkSize))
 	}
 	b.mu.Unlock()
 }