feat: implement nixos-exporter

Prometheus exporter for NixOS-specific metrics including:
- Generation collector: count, current, booted, age, config mismatch
- Flake collector: input age, input info, revision behind

Includes NixOS module, flake packaging, and documentation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

collector/generation_test.go

@@ -0,0 +1,178 @@
+package collector
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestCountGenerations(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create some generation symlinks
+	for _, name := range []string{
+		"system-1-link",
+		"system-2-link",
+		"system-10-link",
+		"system",       // current system link, should not be counted
+		"other-file",   // unrelated file
+		"system-x-link", // malformed, should not be counted
+	} {
+		path := filepath.Join(dir, name)
+		if err := os.Symlink("/nix/store/dummy", path); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	count, err := countGenerations(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if count != 3 {
+		t.Errorf("expected 3 generations, got %d", count)
+	}
+}
+
+func TestGetCurrentGeneration(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create system symlink pointing to a generation
+	if err := os.Symlink("system-42-link", filepath.Join(dir, "system")); err != nil {
+		t.Fatal(err)
+	}
+
+	gen, err := getCurrentGeneration(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if gen != 42 {
+		t.Errorf("expected generation 42, got %d", gen)
+	}
+}
+
+func TestGetBootedGeneration(t *testing.T) {
+	profileDir := t.TempDir()
+	bootedDir := t.TempDir()
+
+	storePath := "/nix/store/abc123-nixos-system"
+
+	// Create generation symlinks
+	if err := os.Symlink("/nix/store/other", filepath.Join(profileDir, "system-1-link")); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(storePath, filepath.Join(profileDir, "system-2-link")); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink("/nix/store/another", filepath.Join(profileDir, "system-3-link")); err != nil {
+		t.Fatal(err)
+	}
+
+	// Create booted-system symlink
+	bootedSystemPath := filepath.Join(bootedDir, "booted-system")
+	if err := os.Symlink(storePath, bootedSystemPath); err != nil {
+		t.Fatal(err)
+	}
+
+	gen, err := getBootedGeneration(profileDir, bootedSystemPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if gen != 2 {
+		t.Errorf("expected booted generation 2, got %d", gen)
+	}
+}
+
+func TestCheckConfigMismatch(t *testing.T) {
+	dir := t.TempDir()
+
+	currentPath := filepath.Join(dir, "current-system")
+	bootedPath := filepath.Join(dir, "booted-system")
+
+	// Same target = no mismatch
+	if err := os.Symlink("/nix/store/same", currentPath); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink("/nix/store/same", bootedPath); err != nil {
+		t.Fatal(err)
+	}
+
+	mismatch, err := checkConfigMismatch(currentPath, bootedPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if mismatch {
+		t.Error("expected no mismatch when targets are the same")
+	}
+
+	// Different targets = mismatch
+	if err := os.Remove(currentPath); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink("/nix/store/different", currentPath); err != nil {
+		t.Fatal(err)
+	}
+
+	mismatch, err = checkConfigMismatch(currentPath, bootedPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !mismatch {
+		t.Error("expected mismatch when targets differ")
+	}
+}
+
+func TestGetGenerationAge(t *testing.T) {
+	dir := t.TempDir()
+
+	// Create system symlink
+	if err := os.Symlink("system-1-link", filepath.Join(dir, "system")); err != nil {
+		t.Fatal(err)
+	}
+
+	age, err := getGenerationAge(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Age should be very small since we just created it
+	if age < 0 || age > 1 {
+		t.Errorf("expected age close to 0, got %f", age)
+	}
+}
+
+func TestGenerationPattern(t *testing.T) {
+	tests := []struct {
+		name    string
+		match   bool
+		genNum  string
+	}{
+		{"system-1-link", true, "1"},
+		{"system-42-link", true, "42"},
+		{"system-123-link", true, "123"},
+		{"system", false, ""},
+		{"system-link", false, ""},
+		{"system--link", false, ""},
+		{"system-abc-link", false, ""},
+		{"other-1-link", false, ""},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			matches := generationPattern.FindStringSubmatch(tt.name)
+			if tt.match {
+				if matches == nil {
+					t.Errorf("expected %q to match", tt.name)
+				} else if matches[1] != tt.genNum {
+					t.Errorf("expected generation %q, got %q", tt.genNum, matches[1])
+				}
+			} else {
+				if matches != nil {
+					t.Errorf("expected %q not to match", tt.name)
+				}
+			}
+		})
+	}
+}