Adds a new json_timestamp_units configuration parameter (#2587)

plugins/serializers/json/json.go

@@ -2,19 +2,27 @@ package json
 
 import (
 	ejson "encoding/json"
+	"time"
 
 	"github.com/influxdata/telegraf"
 )
 
 type JsonSerializer struct {
+	TimestampUnits time.Duration
 }
 
 func (s *JsonSerializer) Serialize(metric telegraf.Metric) ([]byte, error) {
 	m := make(map[string]interface{})
+	units_nanoseconds := s.TimestampUnits.Nanoseconds()
+	// if the units passed in were less than or equal to zero,
+	// then serialize the timestamp in seconds (the default)
+	if units_nanoseconds <= 0 {
+		units_nanoseconds = 1000000000
+	}
 	m["tags"] = metric.Tags()
 	m["fields"] = metric.Fields()
 	m["name"] = metric.Name()
-	m["timestamp"] = metric.UnixNano() / 1000000000
+	m["timestamp"] = metric.UnixNano() / units_nanoseconds
 	serialized, err := ejson.Marshal(m)
 	if err != nil {
 		return []byte{}, err