[NEW] Add keyspace_hit_ratio metric in info stats

Regarding cache hits and misses, you can check them through the info stats command in valkey.
There is keyspace_hits(cache hit) and keyspace_misses(cache miss) information, but there is no keyspace_hit_ratio metric.

I would like some additional comments regarding the keyspace_hit_ratio metric. Please give me feedback.

Please see #498 for details.