[Feature] Track disk usage with dstack stats

[peterschmidt85]

Problem:
Disk space is one of the most important factors when working with large models. Available disk space may depend on what is already downloaded, on its cache, etc. As I user I'd like to see how much space is available to understand if more space must be requested or if I need to delete cache, etc.

Solution:
Add DISK used/total to dstack stats`.

[r4victor]

Note that getting container disk usage can be an expensive operation (e.g. simply calling docker ps --size can take minutes). So we need to look for an efficient mechanism to report disk usage. So here's, for example, Kubernetes (cadvisor) increased disk metrics interval to 1 minute: google/cadvisor#910.