emit certificate.expiration_ttl_seconds metric (#782)

interface.go

@@ -380,6 +380,8 @@ func (f *Interface) emitStats(ctx context.Context, i time.Duration) {
 
 	udpStats := udp.NewUDPStatsEmitter(f.writers)
 
+	certExpirationGauge := metrics.GetOrRegisterGauge("certificate.ttl_seconds", nil)
+
 	for {
 		select {
 		case <-ctx.Done():
@@ -388,6 +390,7 @@ func (f *Interface) emitStats(ctx context.Context, i time.Duration) {
 			f.firewall.EmitStats()
 			f.handshakeManager.EmitStats()
 			udpStats()
+			certExpirationGauge.Update(int64(f.certState.Load().certificate.Details.NotAfter.Sub(time.Now()) / time.Second))
 		}
 	}
 }