Add memory usage stats for disk table

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

**Describe the solution you'd like**
rocksdb memory usage can be obtained its APIs: https://github.com/facebook/rocksdb/wiki/Memory-usage-in-RocksDB

The issue is about adding memory usage stats for a disk table, likely using RocksDB APIs. Look for the disk table implementation in the codebase, find where RocksDB is integrated, and use the linked RocksDB memory usage documentation to expose stats. Check existing monitoring or stats code to see how metrics are reported. Verify the change by running any existing tests related to disk tables or monitoring.

Written by the indexing model from the issue text.