expose systemd's MemoryCurrent value in metrics

Signed-off-by: Evgeni Golov <evgeni@golov.de>

systemd/systemd.go

@@ -69,6 +69,7 @@ type Collector struct {
 	unitActiveExitTimeDesc        *prometheus.Desc
 	unitInactiveEnterTimeDesc     *prometheus.Desc
 	unitInactiveExitTimeDesc      *prometheus.Desc
+	unitMemoryCurrentDesc         *prometheus.Desc
 	nRestartsDesc                 *prometheus.Desc
 	timerLastTriggerDesc          *prometheus.Desc
 	socketAcceptedConnectionsDesc *prometheus.Desc
@@ -142,6 +143,11 @@ func NewCollector(logger log.Logger) (*Collector, error) {
 		"Last time the unit transitioned out of the inactive state",
 		[]string{"name", "type"}, nil,
 	)
+	unitMemoryCurrentDesc := prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "unit_memory_current"),
+		"Current memory per Systemd unit",
+		[]string{"name"}, nil,
+	)
 	nRestartsDesc := prometheus.NewDesc(
 		prometheus.BuildFQName(namespace, "", "service_restart_total"),
 		"Service unit count of Restart triggers", []string{"name"}, nil)
@@ -205,6 +211,7 @@ func NewCollector(logger log.Logger) (*Collector, error) {
 		unitActiveExitTimeDesc:        unitActiveExitTimeDesc,
 		unitInactiveEnterTimeDesc:     unitInactiveEnterTimeDesc,
 		unitInactiveExitTimeDesc:      unitInactiveExitTimeDesc,
+		unitMemoryCurrentDesc:         unitMemoryCurrentDesc,
 		nRestartsDesc:                 nRestartsDesc,
 		timerLastTriggerDesc:          timerLastTriggerDesc,
 		socketAcceptedConnectionsDesc: socketAcceptedConnectionsDesc,
@@ -235,6 +242,7 @@ func (c *Collector) Describe(desc chan<- *prometheus.Desc) {
 	desc <- c.unitStartTimeDesc
 	desc <- c.unitTasksCurrentDesc
 	desc <- c.unitTasksMaxDesc
+	desc <- c.unitMemoryCurrentDesc
 	desc <- c.nRestartsDesc
 	desc <- c.timerLastTriggerDesc
 	desc <- c.socketAcceptedConnectionsDesc
@@ -325,6 +333,11 @@ func (c *Collector) collectUnit(conn *dbus.Conn, ch chan<- prometheus.Metric, un
 			level.Warn(logger).Log("msg", errUnitMetricsMsg, "err", err)
 		}
 
+		err = c.collectServiceMemoryMetrics(conn, ch, unit)
+		if err != nil {
+			level.Warn(logger).Log("msg", errUnitMetricsMsg, "err", err)
+		}
+
 		err = c.collectUnitCPUUsageMetrics("Service", conn, ch, unit)
 		if err != nil {
 			level.Warn(logger).Log("msg", errUnitMetricsMsg, "err", err)
@@ -686,6 +699,27 @@ func (c *Collector) collectServiceTasksMetrics(conn *dbus.Conn, ch chan<- promet
 	return nil
 }
 
+func (c *Collector) collectServiceMemoryMetrics(conn *dbus.Conn, ch chan<- prometheus.Metric, unit dbus.UnitStatus) error {
+	memoryCurrentCount, err := conn.GetUnitTypePropertyContext(c.ctx, unit.Name, "Service", "MemoryCurrent")
+	if err != nil {
+		return errors.Wrapf(err, errGetPropertyMsg, "MemoryCurrent")
+	}
+
+	currentCount, ok := memoryCurrentCount.Value.Value().(uint64)
+	if !ok {
+		return errors.Errorf(errConvertUint64PropertyMsg, "MemoryCurrent", memoryCurrentCount.Value.Value())
+	}
+
+	// Don't set if memoryCurrent if dbus reports MaxUint64.
+	if currentCount != math.MaxUint64 {
+		ch <- prometheus.MustNewConstMetric(
+			c.unitMemoryCurrentDesc, prometheus.GaugeValue,
+			float64(currentCount), unit.Name)
+	}
+
+	return nil
+}
+
 func (c *Collector) collectTimerTriggerTime(conn *dbus.Conn, ch chan<- prometheus.Metric, unit dbus.UnitStatus) error {
 	lastTriggerValue, err := conn.GetUnitTypePropertyContext(c.ctx, unit.Name, "Timer", "LastTriggerUSec")
 	if err != nil {