Add storage layer metrics

[josef podany]

In order to improve observability, add metrics for

• table wrapper
  • latency metrics for Range/Put/Delete
  • failure metrics for Get/Set/Delete
• table pebble storage
  • # of write stalls
  • # of compactions
  • compaction sizes
  • DB size
  • Cache usage metrics
• metadata
  • latency metrics for Get/Set/Delete
  • failure metrics for Get/Set/Delete