Allow configuration of timestamp layout

Signed-off-by: Stephen J Day <stephen.day@docker.com>

formatters/logstash/logstash.go

@@ -3,19 +3,27 @@ package logstash
 import (
 	"encoding/json"
 	"fmt"
+
 	"github.com/Sirupsen/logrus"
-	"time"
 )
 
 // Formatter generates json in logstash format.
 // Logstash site: http://logstash.net/
 type LogstashFormatter struct {
 	Type string // if not empty use for logstash type field.
+
+	// TimestampFormat sets the format used for timestamps.
+	TimestampFormat string
 }
 
 func (f *LogstashFormatter) Format(entry *logrus.Entry) ([]byte, error) {
 	entry.Data["@version"] = 1
-	entry.Data["@timestamp"] = entry.Time.Format(time.RFC3339)
+
+	if f.TimestampFormat == "" {
+		f.TimestampFormat = logrus.DefaultTimestampFormat
+	}
+
+	entry.Data["@timestamp"] = entry.Time.Format(f.TimestampFormat)
 
 	// set message field
 	v, ok := entry.Data["message"]