Add Prometheus metrics support to track cache entries size

[dmercuriali]

FEATURE REQUEST

Add Prometheus metrics to the cache client to keep track of each entry size in bytes. This should be easy to achieve by wrapping the 'actualMemory' AtomicLong attribute inside CacheClient.

metric name: something like cacheentry_memory_used
labels: clientid, entrykey
type: gauge

We could also add metrics for client hits, misses and other operations.