feat: add nixos_flake_info metric with current and remote revisions

Add a new info metric that exposes the current system's flake revision
and the latest remote revision as labels. This makes it easier to see
exactly which revision is deployed vs available.

Also adds version constant to Go code and extracts it in flake.nix,
providing a single source of truth for the version.

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

collector/flake.go

@@ -27,6 +27,7 @@ type FlakeCollector struct {
 
 	inputAge       *prometheus.Desc
 	inputInfo      *prometheus.Desc
+	flakeInfo      *prometheus.Desc
 	revisionBehind *prometheus.Desc
 
 	mu          sync.RWMutex
@@ -75,6 +76,11 @@ func NewFlakeCollector(flakeURL string, checkInterval time.Duration) *FlakeColle
 			"Info gauge with revision and type labels",
 			[]string{"input", "rev", "type"}, nil,
 		),
+		flakeInfo: prometheus.NewDesc(
+			"nixos_flake_info",
+			"Info gauge with current and remote flake revisions",
+			[]string{"current_rev", "remote_rev"}, nil,
+		),
 		revisionBehind: prometheus.NewDesc(
 			"nixos_flake_revision_behind",
 			"1 if current system revision differs from remote latest, 0 if match",
@@ -86,6 +92,7 @@ func NewFlakeCollector(flakeURL string, checkInterval time.Duration) *FlakeColle
 func (c *FlakeCollector) Describe(ch chan<- *prometheus.Desc) {
 	ch <- c.inputAge
 	ch <- c.inputInfo
+	ch <- c.flakeInfo
 	ch <- c.revisionBehind
 }
 
@@ -168,14 +175,17 @@ func (c *FlakeCollector) collectRevisionBehind(ch chan<- prometheus.Metric, data
 		return
 	}
 
+	remoteRev := data.Revision
+	if len(remoteRev) > 7 {
+		remoteRev = remoteRev[:7]
+	}
+
+	// Emit flake info metric with revisions
+	ch <- prometheus.MustNewConstMetric(c.flakeInfo, prometheus.GaugeValue, 1, currentRev, remoteRev)
+
 	behind := 0.0
 	if currentRev != "" && data.Revision != "" {
-		// Compare short hashes
-		remoteShort := data.Revision
-		if len(remoteShort) > 7 {
-			remoteShort = remoteShort[:7]
-		}
-		if currentRev != remoteShort && !strings.HasPrefix(data.Revision, currentRev) {
+		if currentRev != remoteRev && !strings.HasPrefix(data.Revision, currentRev) {
 			behind = 1.0
 		}
 	}