metrics: supply the metric on storage usage.

[LykxSassinator]

Description

This pr is used to supply the necessary metric on storage usage of engine.

[LykxSassinator]

/cc @tabokie PTAL, thx.

[tabokie]

Why is this needed? raft_engine_log_file_count * file_size should be equivalent. If needed you can publish file_size via a separate counter.

[LykxSassinator]

Why is this needed? raft_engine_log_file_count * file_size should be equivalent. If needed you can publish file_size via a separate counter.

From users, they think the usage of raft-engine is not clear as expected. They expect the storage size can be easily checked.

[tabokie]

Why is this needed? raft_engine_log_file_count * file_size should be equivalent. If needed you can publish file_size via a separate counter.

From users, they think the usage of raft-engine is not clear as expected. They expect the storage size can be easily checked.

You can either add a counter for raft-engine file_size config in TiKV, then use file_count * file_size to display the size. Or manually flush the metrics in TiKV's EngineMetricsManager. The information is already there, I don't think it's worth adding another counter to raft-engine.