systemd.go: Added systemd health metric

Fixes: prometheus-community#112

Signed-off-by: Jonathan Davies <[email protected]>

systemd/systemd.go

@@ -58,6 +58,7 @@ var (
 type Collector struct {
 	ctx                           context.Context
 	logger                        log.Logger
+	healthy                       *prometheus.Desc
 	unitCPUTotal                  *prometheus.Desc
 	unitState                     *prometheus.Desc
 	unitInfo                      *prometheus.Desc
@@ -84,6 +85,10 @@ type Collector struct {
 
 // NewCollector returns a new Collector exposing systemd statistics.
 func NewCollector(logger log.Logger) (*Collector, error) {
+	healthy := prometheus.NewDesc(
+		prometheus.BuildFQName(namespace, "", "healthy"),
+		"Systemd exporter DBus connection health", nil, nil,
+	)
 	// Type is labeled twice e.g. name="foo.service" and type="service" to maintain compatibility
 	// with users before we started exporting type label
 	unitState := prometheus.NewDesc(
@@ -194,6 +199,7 @@ func NewCollector(logger log.Logger) (*Collector, error) {
 	return &Collector{
 		ctx:                           ctx,
 		logger:                        logger,
+		healthy:                       healthy,
 		unitCPUTotal:                  unitCPUTotal,
 		unitState:                     unitState,
 		unitInfo:                      unitInfo,
@@ -223,11 +229,14 @@ func (c *Collector) Collect(ch chan<- prometheus.Metric) {
 	err := c.collect(ch)
 	if err != nil {
 		level.Error(c.logger).Log("msg", "error collecting metrics", "err", err)
+		ch <- prometheus.MustNewConstMetric(
+			c.healthy, prometheus.GaugeValue, 0)
 	}
 }
 
 // Describe gathers descriptions of Metrics
 func (c *Collector) Describe(desc chan<- *prometheus.Desc) {
+	desc <- c.healthy
 	desc <- c.unitCPUTotal
 	desc <- c.unitState
 	desc <- c.unitInfo
@@ -282,6 +291,10 @@ func (c *Collector) collect(ch chan<- prometheus.Metric) error {
 	}
 
 	wg.Wait()
+
+	ch <- prometheus.MustNewConstMetric(
+		c.healthy, prometheus.GaugeValue, 1)
+
 	return nil
 }