fix(battery): handle no batteries being present

battery/ioreg.go

@@ -3,6 +3,7 @@
 package battery
 
 import (
+	"bytes"
 	"os/exec"
 
 	"howett.net/plist"
@@ -39,6 +40,11 @@ func getAllRaw() ([]*batteryRaw, error) {
 	}
 
 	batteries := []*batteryRaw{}
+
+	if len(bytes.TrimSpace(b)) == 0 {
+		return batteries, nil
+	}
+
 	_, err = plist.Unmarshal(b, &batteries)
 	if err != nil {
 		return nil, err

prom/collector.go

@@ -17,7 +17,9 @@ const (
 )
 
 type Collector struct {
-	descInfo                       *prometheus.Desc
+	descInfo         *prometheus.Desc
+	descBatteryCount *prometheus.Desc
+
 	descBatteryCellDisconnectCount *prometheus.Desc
 	descChargeRateAmps             *prometheus.Desc
 	descChargeRateWatts            *prometheus.Desc
@@ -47,6 +49,14 @@ func NewCollector(namespace string) *Collector {
 			[]string{serialLabel, deviceNameLabel, builtInLabel},
 			nil,
 		),
+		descBatteryCount: prometheus.NewDesc(
+			prometheus.BuildFQName(
+				namespace, "battery", "count",
+			),
+			"Total number of batteries.",
+			nil,
+			nil,
+		),
 		descBatteryCellDisconnectCount: prometheus.NewDesc(
 			prometheus.BuildFQName(
 				namespace, "battery", "cell_disconnect_count",
@@ -191,6 +201,7 @@ func NewCollector(namespace string) *Collector {
 
 func (c *Collector) Describe(ch chan<- *prometheus.Desc) {
 	ch <- c.descInfo
+	ch <- c.descBatteryCount
 	ch <- c.descBatteryCellDisconnectCount
 	ch <- c.descChargeRateAmps
 	ch <- c.descChargeRateWatts
@@ -220,6 +231,12 @@ func (c *Collector) Collect(ch chan<- prometheus.Metric) {
 		return
 	}
 
+	ch <- prometheus.MustNewConstMetric(
+		c.descBatteryCount,
+		prometheus.GaugeValue,
+		float64(len(batteries)),
+	)
+
 	for _, battery := range batteries {
 		labels := []string{battery.Serial}