Add system uptime metric, string formatted AND in float64

closes #150

plugins/system/system.go

@@ -1,39 +1,48 @@
 package system
 
-import "github.com/influxdb/telegraf/plugins"
+import (
+	"github.com/cloudfoundry/gosigar"
 
-type SystemStats struct {
-	ps PS
-}
+	"github.com/influxdb/telegraf/plugins"
+)
+
+type SystemStats struct{}
 
 func (_ *SystemStats) Description() string {
-	return "Read metrics about system load"
+	return "Read metrics about system load & uptime"
 }
 
 func (_ *SystemStats) SampleConfig() string { return "" }
 
-func (s *SystemStats) add(acc plugins.Accumulator,
+func (_ *SystemStats) add(acc plugins.Accumulator,
 	name string, val float64, tags map[string]string) {
 	if val >= 0 {
 		acc.Add(name, val, tags)
 	}
 }
 
-func (s *SystemStats) Gather(acc plugins.Accumulator) error {
-	lv, err := s.ps.LoadAvg()
-	if err != nil {
+func (_ *SystemStats) Gather(acc plugins.Accumulator) error {
+	loadavg := sigar.LoadAverage{}
+	if err := loadavg.Get(); err != nil {
 		return err
 	}
 
-	acc.Add("load1", lv.Load1, nil)
-	acc.Add("load5", lv.Load5, nil)
-	acc.Add("load15", lv.Load15, nil)
+	uptime := sigar.Uptime{}
+	if err := uptime.Get(); err != nil {
+		return err
+	}
+
+	acc.Add("load1", loadavg.One, nil)
+	acc.Add("load5", loadavg.Five, nil)
+	acc.Add("load15", loadavg.Fifteen, nil)
+	acc.Add("uptime", uptime.Length, nil)
+	acc.Add("uptime_format", uptime.Format(), nil)
 
 	return nil
 }
 
 func init() {
 	plugins.Add("system", func() plugins.Plugin {
-		return &SystemStats{ps: &systemPS{}}
+		return &SystemStats{}
 	})
 }