Support for providing quota metrics

[xiaogaozi]

What would you like to be added:

Add quota-related metrics to the Promethus API, such as quota usage of a directory (including data usage and inodes usage).

Why is this needed:

Enhance the observability of JuiceFS

[polyrabbit]

Better to also emit space/inode capacity of this quota, so user can set an accurate usage alert. eg. https://github.com/juicedata/juicefs/pull/5424