Add lifetime to certificates monitored

tlsconmon/tlsconmon.go

@@ -37,6 +37,13 @@ var (
 		Help:      "Error checking the certificate.",
 	}, []string{"address"})
 
+	gaugeCertLifetime = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "labmon",
+		Subsystem: "tlsconmon",
+		Name:      "certificate_lifetime_seconds",
+		Help:      "How long the certificate is valid in seconds.",
+	}, []string{"address"})
+
 	// OTEL tracing
 	tracer = otel.Tracer(name)
 )
@@ -104,6 +111,8 @@ func (tm *TLSConnectionMonitor) Start(ctx context.Context) {
 	if err := tm.fetchCert(ctx); err != nil {
 		gaugeCertError.WithLabelValues(tm.Address).Set(1)
 		gaugeCertTimeLeft.WithLabelValues(tm.Address).Set(0)
+		lifetime := tm.cert.NotAfter.Sub(tm.cert.NotBefore).Seconds()
+		gaugeCertLifetime.WithLabelValues(tm.Address).Set(lifetime)
 	} else {
 		gaugeCertError.WithLabelValues(tm.Address).Set(0)
 		timeLeft := time.Until(tm.cert.NotAfter).Seconds()