curvefs/monitor: diskcache num/bytes

[Cyber-SiKu]

Describe the bug (描述bug)

There is a discrepancy between the monitoring item display and the actual.

There are already 8565550 files in the cache disk, with a content of 50GiB.

To Reproduce (复现方法)

warmup file or directory.

Expected behavior (期望行为)

The monitoring items are displayed the same as the actual ones.

Versions (各种版本)
OS:
Compiler:
branch:
commit id:

Additional context/screenshots (更多上下文/截图)

cache disk:

[wuhongsong]

@wuhongsong