code: Add metrics to the block store

[romac]

Metrics

/// Size of the database database (bytes)
db_size: Gauge,

/// Amount of data written to the database (bytes)
db_write_bytes: Counter,

/// Amount of data read from the database (bytes)
db_read_bytes: Counter,

/// Amount of key data read from the database (bytes)
db_key_read_bytes: Counter,

/// Total number of reads from the database
db_read_count: Counter,

/// Total number of writes to the database
db_write_count: Counter,

/// Total number of deletions to the database
db_delete_count: Counter,

/// Time taken to read from the database (seconds)
db_read_time: Histogram,

/// Time taken to write to the database (seconds)
db_write_time: Histogram,

/// Time taken to delete from the database (seconds)
db_delete_time: Histogram,

[romac]

Work in progress in #636