feat: metrics for state & block store caches

[frrist]

Gives us metrics like:

# HELP visor_state_store_cache_hits Number of cache hits for the state store cache
# TYPE visor_state_store_cache_hits counter
visor_state_store_cache_hits 8.774107e+06

# HELP visor_state_store_cache_limit Max size of cache
# TYPE visor_state_store_cache_limit gauge
visor_state_store_cache_limit 500000

# HELP visor_state_store_cache_read Number of reads from the state store cache
# TYPE visor_state_store_cache_read counter
visor_state_store_cache_read 1.7223509e+07

# HELP visor_state_store_cache_size Number of elements in the cache
# TYPE visor_state_store_cache_size gauge
visor_state_store_cache_size 500000

and

# HELP visor_block_store_cache_hits Number of cache hits for the block store cache
# TYPE visor_block_store_cache_hits counter
visor_block_store_cache_hits 254849

# HELP visor_block_store_cache_limit Max size of cache
# TYPE visor_block_store_cache_limit gauge
visor_block_store_cache_limit 500000

# HELP visor_block_store_cache_read Number of reads from the block store cache
# TYPE visor_block_store_cache_read counter
visor_block_store_cache_read 8.760208e+06

# HELP visor_block_store_cache_size Number of elements in the cache
# TYPE visor_block_store_cache_size gauge
visor_block_store_cache_size 500000

[birdychang]

Thanks!