Fix bug (#905)

plugins/metrics/process.go

@@ -24,7 +24,7 @@ func measureCPUUsage() {
 	// Percent calculates the percentage of cpu used either per CPU or combined.
 	// TODO: use func PercentWithContext for more detailed info.
 	percent, err := cpu.Percent(time.Second, false)
-	if err == nil {
+	if err == nil && len(percent) > 0 {
 		p = percent[0]
 	}
 	metrics.Events().CPUUsage.Trigger(p)