Stats/Metrics tracking db operations

[zaidoon1]

Similar to rocksdb, it would be nice if we have access to various stats about the db: https://github.com/facebook/rocksdb/wiki/Statistics

things like:

• db size
• number of sst files
• number of kvs
• number of tombstones
• number of levels
• number of memtables, their current size
• number of entries in memtables, number of tombstones
• number of flushes
• number of compactions happening
• usefulness of bloom filters (HITs vs MISS), how many operations to disk were filtered out, how many operations to disk were not filtered out
• etc..

[marvin-j97]

Some of these are already available:

• db size: https://docs.rs/fjall/latest/fjall/struct.Keyspace.html#method.disk_space
• number of kvs: https://docs.rs/fjall/latest/fjall/struct.PartitionHandle.html#method.approximate_len

and some are not exposed yet, but technically exist:

• number of disk segments: https://docs.rs/lsm-tree/latest/lsm_tree/trait.AbstractTree.html#tymethod.segment_count
• number of L0 segments: https://docs.rs/lsm-tree/latest/lsm_tree/trait.AbstractTree.html#tymethod.first_level_segment_count
• size of bloom filters: https://github.com/fjall-rs/lsm-tree/blob/c7164d240c51ec3cbecb32f841df2f0b5b05a0c8/src/abstract.rs#L24
• segment stats that could be aggregated: https://github.com/fjall-rs/lsm-tree/blob/c7164d240c51ec3cbecb32f841df2f0b5b05a0c8/src/segment/meta/mod.rs#L26-L78

Tracking block cache and bloom filter (and maybe file descriptor cache) hits/misses should probably be behind a feature flag