prometheus: convert zrepl_version_daemon to zrepl_start_time metric

closes https://github.com/zrepl/zrepl/pull/556
fixes #553

dist/grafana/grafana-prometheus-zrepl.json

@@ -249,7 +249,7 @@
             "stack": true,
             "steppedLine": false,
             "targets": [{
-                "expr": "zrepl_version_daemon{job='$prom_job_name'}",
+                "expr": "sgn(zrepl_start_time{job='$prom_job_name'})",
                 "format": "time_series",
                 "interval": "",
                 "intervalFactor": 1,

docs/changelog.rst

@@ -16,6 +16,17 @@ Changelog
 The changelog summarizes bugfixes that are deemed relevant for users and package maintainers.
 Developers should consult the git commit log or GitHub issue tracker.
 
+0.6 (Unreleased)
+----------------
+
+* `Feature Wishlist on GitHub <https://github.com/zrepl/zrepl/discussions/547>`_
+
+* |break| |feature| convert Prometheus metric ``zrepl_version_daemon`` to ``zrepl_start_time`` metric
+
+  * The metric still reports the zrepl version in a label.
+    But the metric *value* is now the Unix timestamp at the time the daemon was started.
+    The Grafana dashboard in :repomasterlink:`dist/grafana` has been updated.
+
 0.5
 ---
 

version/version.go

@@ -34,20 +34,19 @@ func (i *ZreplVersionInformation) String() string {
 		i.Version, i.RuntimeGo, i.RuntimeGOOS, i.RuntimeGOARCH, i.RUNTIMECompiler)
 }
 
-var prometheusMetric = prometheus.NewUntypedFunc(
-	prometheus.UntypedOpts{
+var prometheusMetric = prometheus.NewGauge(
+	prometheus.GaugeOpts{
 		Namespace: "zrepl",
-		Subsystem: "version",
-		Name:      "daemon",
-		Help:      "zrepl daemon version",
+		Name:      "start_time",
+		Help:      "zrepl daemon start time and version",
 		ConstLabels: map[string]string{
 			"raw":          zreplVersion,
 			"version_info": NewZreplVersionInformation().String(),
 		},
 	},
-	func() float64 { return 1 },
 )
 
 func PrometheusRegister(r prometheus.Registerer) {
 	r.MustRegister(prometheusMetric)
+	prometheusMetric.SetToCurrentTime()
 }