core,triedb: add fastcache metrics

[jsvisa]

Add more metrics about the fastcache, so the users have a clear understanding of the use of cache and know how to configure the size of each cache.

I'm running with

geth 
--cache=6144 # 6GB
--cache.database=20 # 20%
--cache.snapshot=20 # 20%
--cache.gc=5        # 5%
--cache.trie=40     # 40%

This is the metrics in grafana dashboard:

I can see that the disk layer cache(--cache.snapshot) is not used much, given 6*0.2=1.2GB, but only 30% was in use; and for the trie cache(--cache.trie) is fully used, so in my case, I now know I need to reduce the cache used for snapshot, and increase it for trie.