Add metrics for metadata directory I/O

There's good metrics for block manager (data dir) and WAL operations, like block_manager_total_bytes_written, block_manager_total_bytes_read, log_bytes_logged }}, and the {{log_append_latency histogram. What we are missing are metrics about the amount of metadata I/O. It'd be nice to add

• metadata_bytes_read
• metadata_bytes_written
• latency histograms for bytes read and bytes written