[leo_backend_db] Implement status callback for eleveldb

[mocchira]

In order to make it easy to debug/monitor I/O activity inside leveldb in detail like the following case.

• [leo_storage] Unresponsive when Disk IO takes a long time #503
• "badarg" from watchdog on storage node in error log #644

Providing (Disk Throughput/The number of files) per LSM level using https://github.com/basho/leveldb/blob/2.0.33/db/db_impl.cc#L2113-L2135

If the above information is not enough then also considering to provide whether there is any scheduled compaction using https://github.com/basho/leveldb/blob/2.0.33/db/db_impl.cc#L2404-L2412
We have to add a new I/F in eleveldb in this case (and maybe have to maintain our home grown eleveldb).

Reference: #503 (comment)

[mocchira]

@yosukehara
I noticed that leo_backend_db_server:status has been used for providing the number of entries stored in a leo_backend_db at https://github.com/leo-project/leo_mq/blob/1.4.13/src/leo_mq_api.erl#L160-L167.
As changing the behavior of leo_backend_db_server:status also requires the changes of leo_mq_api:status,
I think it would be good to implement this feature as another callback function like compaction_status rather than changing the current status semantics. Thoughts?

[yosukehara]

I think it would be good to implement this feature as another callback function like compaction_status rather than changing the current status semantics. Thoughts?

@mocchira I agree with your opinion. Let's implement its function.

[mocchira]

@yosukehara thanks. will do with compaction_status.

[yosukehara]

Fixed