feat: Add disk usage percentage and warn on high usage (#57)

* fix: Log GID instead of UID in user GID tracing field

The tracing statement for `user.gid` was reading from `user.uid`
instead of `user.gid`, causing the wrong value to be reported.

* refactor: Use idiomatic empty check for disk collection

Replace `into_iter().next().is_none()` with `list().is_empty()`
for clarity, and use `list().iter()` for the actual collection.

* fix: Remove stale .source() call with unused result

This was likely a debugging leftover — the error source chain is
already captured via the `successors` iterator below.

* fix: Fix typo "proess" -> "process" in error message

* fix: Replace unwrap() calls with error logging in collection loop

JSON serialization and file write can fail at runtime (e.g. disk
full). Log the error and continue the loop instead of crashing,
since this tool may run continuously for hours.

* fix: Use tokio::time::sleep instead of std::thread::sleep

std::thread::sleep blocks the entire tokio worker thread.
Since main is already async, use the non-blocking alternative.

* fix: Handle DNS resolver initialization failure gracefully

In a container debugging tool, broken DNS config (/etc/resolv.conf)
is a likely scenario to diagnose. Log the error and skip DNS lookups
instead of panicking.

* fix: Wrap network collector in ComponentResult for consistent error handling

The network collector silently swallowed interface listing errors by
returning empty data. Now it returns Result so the orchestrator wraps
it in ComponentResult, matching the pattern used by other fallible
collectors. Errors appear in JSON output instead of being silently
lost.

* refactor: Use BTreeMap for deterministic JSON key ordering

HashMap produces non-deterministic JSON output, making it hard to
diff containerdebug output across runs. BTreeMap sorts keys
consistently.

* feat: Add disk usage percentage and warn on high usage

Add `usage_percent` field to disk collection output. When a disk
exceeds 85% usage, log at WARN level instead of INFO so it stands
out in log aggregation systems.

* Apply review suggestion

Author: lfrancke

src/system_information/disk.rs

@@ -5,7 +5,9 @@ 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,
 }
 
 impl Disk {
@@ -21,19 +23,46 @@ impl Disk {
 
 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();
+        // There shouldn't 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: sysinfo_disk.total_space(),
-            available_space: sysinfo_disk.available_space(),
+            total_space,
+            used_space,
+            available_space,
+            usage_percent,
         };
-        tracing::info!(
-            disk.mount_point,
-            disk.name,
-            disk.space.total = disk.total_space,
-            disk.space.available = disk.available_space,
-            "found disk"
-        );
+
+        if usage_percent >= 85.0 {
+            tracing::warn!(
+                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"
+            );
+        } else {
+            tracing::info!(
+                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"
+            );
+        }
         disk
     }
 }