feat: Add disk usage percentage and warn on high usage

[lfrancke]

Summary

• Add usage_percent field to disk collection output, calculated as (total - available) / total * 100
• Log at WARN level when any disk exceeds 85% usage, making it easy to spot in Graylog/Vector

Motivated by a customer running out of space on an attached PVC over the weekend.

Test plan

• cargo test --all-features passes
• cargo clippy with RUSTFLAGS="-D warnings" passes

[sbernauer]

I would like to propose an stylistic improvement (and safety guard). It also stores the usage bytes, and I figured "why not report that as well?"

diff --git a/src/system_information/disk.rs b/src/system_information/disk.rs
index 1cbc429..aadf840 100644
--- a/src/system_information/disk.rs
+++ b/src/system_information/disk.rs
@@ -5,6 +5,7 @@ pub struct Disk {
     pub name: String,
     pub mount_point: String,
     pub total_space: u64,
+    pub used_space: u64,
     pub available_space: u64,
     pub usage_percent: f64,
 }
@@ -24,16 +25,19 @@ impl From<&sysinfo::Disk> for Disk {
     fn from(sysinfo_disk: &sysinfo::Disk) -> Self {
         let total_space = sysinfo_disk.total_space();
         let available_space = sysinfo_disk.available_space();
-        let usage_percent = if total_space > 0 {
-            (total_space - available_space) as f64 / total_space as f64 * 100.0
-        } else {
-            0.0
+        // There should'nt be negative used bytes. We prevent underflow, to not falsely report more
+        // used than total space.
+        let used_space = total_space.saturating_sub(available_space);
+        let usage_percent = match used_space {
+            0 => 0.0,
+            used_space => used_space as f64 / total_space as f64 * 100.0,
         };
 
         let disk = Disk {
             name: sysinfo_disk.name().to_string_lossy().into_owned(),
             mount_point: sysinfo_disk.mount_point().to_string_lossy().into_owned(),
             total_space,
+            used_space,
             available_space,
             usage_percent,
         };
@@ -43,6 +47,7 @@ impl From<&sysinfo::Disk> for Disk {
                 disk.mount_point,
                 disk.name,
                 disk.space.total = disk.total_space,
+                disk.space.used = disk.used_space,
                 disk.space.available = disk.available_space,
                 disk.space.usage_percent = format!("{:.1}%", disk.usage_percent),
                 "disk usage high"
@@ -52,6 +57,7 @@ impl From<&sysinfo::Disk> for Disk {
                 disk.mount_point,
                 disk.name,
                 disk.space.total = disk.total_space,
+                disk.space.used = disk.used_space,
                 disk.space.available = disk.available_space,
                 disk.space.usage_percent = format!("{:.1}%", disk.usage_percent),
                 "found disk"