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>

CLAUDE.md

@@ -90,6 +90,10 @@ Follow semantic versioning:
 - **Minor** (0.x.0): Non-breaking changes adding features
 - **Major** (x.0.0): Breaking changes
 
+Update the `const version` in `main.go`. The Nix build extracts the version from there automatically.
+
+**When to bump**: If any Go code has changed, bump the version before committing. Do this automatically when asked to commit. On feature branches, only bump once per branch (check if version has already been bumped compared to master).
+
 ## Key Design Decisions
 
 - Go chosen for mature Prometheus client library and static binary output

README.md

@@ -80,6 +80,7 @@ services.prometheus.exporters.nixos = {
 |--------|------|--------|-------------|
 | `nixos_flake_input_age_seconds` | Gauge | `input` | Age of flake input in seconds |
 | `nixos_flake_input_info` | Gauge | `input`, `rev`, `type` | Info gauge with revision and type labels |
+| `nixos_flake_info` | Gauge | `current_rev`, `remote_rev` | Info gauge with current and remote flake revisions |
 | `nixos_flake_revision_behind` | Gauge | | 1 if current system revision differs from remote latest |
 
 ## Example Prometheus Alerts
@@ -129,7 +130,7 @@ groups:
 
 ## Known Limitations
 
-- The `nixos_flake_revision_behind` metric relies on parsing the git hash from `/run/current-system/nixos-version`. The format of this file varies depending on NixOS configuration:
+- The `nixos_flake_info` and `nixos_flake_revision_behind` metrics rely on parsing the git hash from `/run/current-system/nixos-version`. The format of this file varies depending on NixOS configuration:
   - Standard format: `25.11.20260203.e576e3c`
   - Custom format: `1994-294a625`
 

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
 		}
 	}

flake.nix

@@ -9,6 +9,12 @@
     let
       supportedSystems = [ "x86_64-linux" "aarch64-linux" ];
       forAllSystems = nixpkgs.lib.genAttrs supportedSystems;
+
+      # Extract version from main.go
+      mainGo = builtins.readFile ./main.go;
+      lines = builtins.filter (x: builtins.isString x) (builtins.split "\n" mainGo);
+      versionLine = builtins.head (builtins.filter (line: builtins.match ".*const version = .*" line != null) lines);
+      version = builtins.head (builtins.match ".*\"([0-9.]+)\".*" versionLine);
     in
     {
       packages = forAllSystems (system:
@@ -18,7 +24,7 @@
         {
           default = pkgs.buildGoModule {
             pname = "nixos-exporter";
-            version = "0.1.0";
+            inherit version;
             src = ./.;
             vendorHash = "sha256-NnvB20rORPS5QF5enbb5KpWaKZ70ybSgfd7wjk21/Cg=";
 

main.go

@@ -15,6 +15,8 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+const version = "0.2.0"
+
 func main() {
 	cfg, err := config.Parse()
 	if err != nil {
@@ -59,7 +61,7 @@ func main() {
 	defer stop()
 
 	go func() {
-		slog.Info("Starting server", "addr", cfg.ListenAddr)
+		slog.Info("Starting server", "version", version, "addr", cfg.ListenAddr)
 		if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
 			slog.Error("Server error", "error", err)
 			os.Exit(1)