Add pending_updates collector

This collector executes a command which is expected to return
pending updates for the system as <all>;<security>.

There is unfortunately no standardized way to pending updates for a
given distribution. Not even across deb based systems. Instead of
implementing a specific collector for deb based systems by jungling
around apt-get / apt-cache commands, this just defers the collection of
the actual updates to a script etc which returns <all>;<security>.

For ubuntu such script is provided by update-notifier-common which is
set as default here.

collector/pending_updates.go

+// +build !nopending_updates
+
+package collector
+
+import (
+	"flag"
+	"fmt"
+	"os/exec"
+	"strconv"
+	"strings"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const (
+	defaultCheckCommand = "/usr/lib/update-notifier/apt-check"
+)
+
+var (
+	checkCommand   = flag.String("checkUpdatesCommand", defaultCheckCommand, "command to run, returning pending updates as <all>;<security>")
+	pendingUpdates = prometheus.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Namespace: Namespace,
+			Name:      "pending_updates",
+			Help:      "Number of pending updates.",
+		}, []string{"type"})
+)
+
+type pendingUpdatesCollector struct{}
+
+func init() {
+	Factories["pending_updates"] = NewPendingUpdatesCollector
+}
+
+// NewPendingUpdatesCollector returns a newly allocated pendingUpdatesCollector.
+// It exposes the number of pending package updates.
+func NewPendingUpdatesCollector(config Config) (Collector, error) {
+	c := pendingUpdatesCollector{}
+	if _, err := prometheus.RegisterOrGet(pendingUpdates); err != nil {
+		return nil, err
+	}
+	return &c, nil
+}
+
+// Update gathers pending updates, implements Collector interface
+func (c *pendingUpdatesCollector) Update() (int, error) {
+	out, err := exec.Command(*checkCommand).CombinedOutput()
+	if err != nil {
+		return 0, err
+	}
+	fields := strings.Split(string(out), ";")
+	if len(fields) < 2 {
+		return 0, fmt.Errorf("Expected %s to return pending updates as <all>;<security> but got: %s", *checkCommand, string(out))
+	}
+	allUpdates, err := strconv.Atoi(fields[0])
+	if err != nil {
+		return 0, err
+	}
+	secUpdates, err := strconv.Atoi(fields[1])
+	if err != nil {
+		return 0, err
+	}
+	pendingUpdates.WithLabelValues("all").Set(float64(allUpdates))
+	pendingUpdates.WithLabelValues("security").Set(float64(secUpdates))
+	return 2, nil
+}