Add CPU count in metrics (#253)

so that the resource metrics collector can use it to calculate
"percentage" load (load * 100 / cpu-count)

metrics.go

@@ -640,6 +640,7 @@ type CPUMetrics struct {
 
 	TimesStat *cpu.TimesStat `json:"timesStat"`
 	LoadStat  *load.AvgStat  `json:"loadStat"`
+	CPUCount  int            `json:"cpuCount"`
 }
 
 // Merge other into 'm'.