add metrics for vinyl.cache

[Satbek]

It is currently difficult to evaluate the efficiency of tuple_cache and whether increasing it makes sense.

Let's add some new metrics for vinyl cache:

• eviction count (index:stat().cache.evict)
• cache hit (index:stat().cache.get)
• cache miss (index:stat().get - index:stat().cache.get)

Also there might be some another useful metrics in index:stat().

Ref: tarantool/tarantool#11859