Add response time stats

stats/percentile.go

@@ -1,6 +1,7 @@
 package stats
 
 import (
+	"encoding/json"
 	"math"
 	"sort"
 	"sync/atomic"
@@ -78,6 +79,10 @@ func (p *Percentile) index() int64 {
 	return idx
 }
 
+func (p *Percentile) MarshalJSON() ([]byte, error) {
+	return json.Marshal(p.Value())
+}
+
 func round(value float64) int64 {
 	if value < 0.0 {
 		value -= 0.5