Add metric representing bird socket query result (#82)

A new gauge metric named `bird_socket_query_success` holds the result of
querying the bird socket for protocol status. If set to 1, the socket
was queried successfully. If the query fails for any of the configured
sockets (IPv4 / IPv6), the value of this metric will be 0.

Co-authored-by: Jan Kral <[email protected]>

metric_collector.go

@@ -76,7 +76,17 @@ func exportersForDefault(c *client.BirdClient, descriptionLabels bool) map[proto
 	}
 }
 
+var socketQueryDesc = prometheus.NewDesc(
+	"bird_socket_query_success",
+	"Result of querying bird socket: 0 = failed, 1 = suceeded",
+	nil,
+	nil,
+)
+
 func (m *MetricCollector) Describe(ch chan<- *prometheus.Desc) {
+
+	ch <- socketQueryDesc
+
 	for _, v := range m.exporters {
 		for _, e := range v {
 			e.Describe(ch)
@@ -85,7 +95,15 @@ func (m *MetricCollector) Describe(ch chan<- *prometheus.Desc) {
 }
 
 func (m *MetricCollector) Collect(ch chan<- prometheus.Metric) {
+
 	protocols, err := m.client.GetProtocols()
+
+	var queryResult float64 = 1
+	if err != nil {
+		queryResult = 0
+	}
+	ch <- prometheus.MustNewConstMetric(socketQueryDesc, prometheus.GaugeValue, queryResult)
+
 	if err != nil {
 		log.Errorln(err)
 		return