Redesign and clean up store metrics

[michaelsproul]

Description

With the transition to atomic database operations, and the removal of various caches, several of the metrics in beacon_node/store/src/metrics.rs have become outdated. For example: DISK_DB_WRITE_BYTES is no longer complete, because it doesn't include writes from atomic transactions, and BEACON_STATE_CACHE_HIT_COUNT is unused since the LRU cache of beacon states was removed. There are a lot of store metrics in that file, and a bunch more are probably wrong/useless as well.

Completing this issue will probably happen naturally when someone next tries to optimise the store. A local grafana instance will be invaluable: https://github.com/sigp/lighthouse-metrics

[paulhauner]

Did you already clear this up @michaelsproul? I vaguely recall something.

[michaelsproul]

Not yet, it's still as it was when this issue was written