use event time for plugin added time

Author: richard-li-sp

lib/fluent/plugin/filter_elasticsearch_timestamp_check.rb

@@ -63,7 +63,7 @@ def filter(tag, time, record)
 
       unless record['fluent_converted_timestamp']
         record['@timestamp'] = record['fluent_added_timestamp'] =
-          Time.now.strftime(@strftime_format)
+          Time.at(time.is_a?(Fluent::EventTime) ? time.to_int : time).strftime(@strftime_format)
         $log.debug("Timestamp added: #{record['@timestamp']}")
       end