Add disk usage stats for disk table

**Is your feature request related to a problem? Please describe.**
Online monitoring

**Describe the solution you'd like**
- Verify current diskused method is applicable to disk table

**Describe alternatives you've considered**
Collect rocksdb disk space usage can also be done: https://github.com/facebook/rocksdb/wiki/Managing-Disk-Space-Utilization

First, locate the existing disk usage method for OpenMLDB's disk table. Then, verify its applicability or integrate RocksDB's disk space usage collection as referenced. Check the RocksDB documentation for relevant APIs. Finally, test the implementation to ensure accurate disk usage stats are reported.

Written by the indexing model from the issue text.