真实数据量

目前不管是config节点还是data节点只有三个指标
```
sys_disk_available_space
sys_disk_free_space
sys_disk_total_space
```
并没有官网上文档介绍的`used_space`

不理解available空间和free空间区别
然后这三者都是反应的数据所处磁盘情况
假如我想了解真实数据量，即～/datanode/data大小无从了解
不管是`sys_disk_total_space-sys_disk_free_space`
还是`sys_disk_total_space-sys_disk_available_space`
都不能真实反应数据量
因为磁盘下可能还有别的目录 别的文件

Start by reviewing the existing sys_disk_available_space, sys_disk_free_space, and sys_disk_total_space metrics exposed by the config and data nodes. Clarify how the documented used_space metric relates to ~/datanode/data and define the expected measurement; done means the reported value reflects that data directory rather than unrelated files on the disk.

Written by the indexing model from the issue text.