Reduce number of global variables used

This is the first step to make the exporter more testable.

collector/time.go

@@ -9,16 +9,9 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
-var (
-	systemTime = prometheus.NewCounter(prometheus.CounterOpts{
-		Namespace: Namespace,
-		Name:      "time",
-		Help:      "System time in seconds since epoch (1970).",
-	})
-)
-
 type timeCollector struct {
 	config Config
+	metric prometheus.Counter
 }
 
 func init() {
@@ -28,17 +21,20 @@ func init() {
 // Takes a config struct and prometheus registry and returns a new Collector exposing
 // the current system time in seconds since epoch.
 func NewTimeCollector(config Config) (Collector, error) {
-	c := timeCollector{
+	return &timeCollector{
 		config: config,
-	}
-
-	return &c, nil
+		metric: prometheus.NewCounter(prometheus.CounterOpts{
+			Namespace: Namespace,
+			Name:      "time",
+			Help:      "System time in seconds since epoch (1970).",
+		}),
+	}, nil
 }
 
 func (c *timeCollector) Update(ch chan<- prometheus.Metric) (err error) {
 	now := time.Now()
 	glog.V(1).Infof("Set time: %f", now.Unix())
-	systemTime.Set(float64(now.Unix()))
-	systemTime.Collect(ch)
+	c.metric.Set(float64(now.Unix()))
+	c.metric.Collect(ch)
 	return err
 }