feat(host_metrics source): add ROOTFS_ROOT support

[fpytloun]

Summary

Adds documented Unix-only ROOTFS_ROOT support to the host_metrics filesystem collector. When set, Vector resolves filesystem capacity and inode reads through the mounted host root while preserving logical mountpoint labels.

The existing PROCFS_ROOT and SYSFS_ROOT startup handling now also treats empty values as unset. Filesystem usage errors include both logical and resolved mount paths.

Vector configuration

sources:
  host:
    type: host_metrics
    collectors: [filesystem]

Run with ROOTFS_ROOT=/mnt/host and mount the host root read-only at /mnt/host.

How did you test this PR?

• cargo test --no-default-features --features sources-host_metrics --lib sources::host_metrics
• cargo fmt --all -- --check
• make check-generated-docs
• make check-changelog-fragments

Focused Clippy was attempted but is blocked by a pre-existing lint under the locally installed Rust 1.96 toolchain: src/config/loading/secret.rs:151 triggers clippy::manual_option_zip. The repository pins Rust 1.95.

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A changelog fragment is included.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Related: containerized host filesystem metrics

[fpytloun]

@pront Hello, can this small change be merged? It solves use-case when vector is running in Docker container with rootfs mounted on eg. /hostfs