docker: collect fs stats for overlayfs storage driver

[bmgeek]

What

Include ContainerdSnapshotterStorageDriver in Docker FsStats() disk usage handling.

Why

#3709 added Docker containerd snapshotter support for the overlayfs storage driver by loading the container rootfs path from the containerd spec.

However, container/docker/fs.go still falls through the default branch for ContainerdSnapshotterStorageDriver, so stats.Filesystem is not populated. As a result, per-container container_fs_* metrics are missing even though the Docker container itself is discovered correctly.

Testing

Manually verified on Docker 29.5+ with storage driver overlayfs:

• before: container_fs_usage_bytes{name!=""} was not emitted
• after: container_fs_usage_bytes, container_fs_limit_bytes, and inode metrics are emitted for Docker containers

Related to #3709 and #3860.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.