[Feature request] More InnoDB buffer pool metrics

[pataquets]

After #18 landing, I would check which ones are missing and I would take the task, if there is interest and no objection to it. I would like all of them to be collected. Currently available on my setup are:

 Innodb_buffer_pool_bytes_data
 Innodb_buffer_pool_bytes_dirty
 Innodb_buffer_pool_dump_status
 Innodb_buffer_pool_load_status
 Innodb_buffer_pool_pages_data
 Innodb_buffer_pool_pages_dirty
 Innodb_buffer_pool_pages_flushed
 Innodb_buffer_pool_pages_free
 Innodb_buffer_pool_pages_lru_flushed
 Innodb_buffer_pool_pages_made_not_young
 Innodb_buffer_pool_pages_made_young
 Innodb_buffer_pool_pages_misc
 Innodb_buffer_pool_pages_old
 Innodb_buffer_pool_pages_total
 Innodb_buffer_pool_read_ahead
 Innodb_buffer_pool_read_ahead_evicted
 Innodb_buffer_pool_read_ahead_rnd
 Innodb_buffer_pool_read_requests
 Innodb_buffer_pool_reads
 Innodb_buffer_pool_wait_free
 Innodb_buffer_pool_write_requests