fix: use configuration-revision for current_rev in flake info metric

The nixos_flake_info metric's current_rev label was incorrectly showing
the nixpkgs input revision (from /run/current-system/nixos-version)
instead of the flake's own revision.

Now reads from /run/current-system/configuration-revision which contains
the flake's self.rev when system.configurationRevision is set in the
NixOS configuration.

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

collector/flake.go

@@ -7,7 +7,6 @@ import (
 	"log/slog"
 	"os"
 	"os/exec"
-	"regexp"
 	"strings"
 	"sync"
 	"time"
@@ -15,11 +14,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-const nixosVersionPath = "/run/current-system/nixos-version"
-
-// revisionPattern extracts the git hash from nixos-version.
-// Formats: "25.11.20260203.e576e3c" or "1994-294a625"
-var revisionPattern = regexp.MustCompile(`[.-]([a-f0-9]{7,40})$`)
+const configRevisionPath = "/run/current-system/configuration-revision"
 
 type FlakeCollector struct {
 	flakeURL      string
@@ -215,18 +210,16 @@ func fetchFlakeMetadata(flakeURL string) (*flakeMetadata, error) {
 }
 
 func getCurrentSystemRevision() (string, error) {
-	data, err := os.ReadFile(nixosVersionPath)
+	data, err := os.ReadFile(configRevisionPath)
 	if err != nil {
+		if os.IsNotExist(err) {
+			// configuration-revision doesn't exist; user hasn't set system.configurationRevision
+			return "", nil
+		}
 		return "", err
 	}
 
-	version := strings.TrimSpace(string(data))
-	matches := revisionPattern.FindStringSubmatch(version)
-	if matches == nil {
-		return "", nil
-	}
-
-	rev := matches[1]
+	rev := strings.TrimSpace(string(data))
 	if len(rev) > 7 {
 		rev = rev[:7]
 	}