Allow to include the event's timestamp as a message tag in the Message encoder

lib/rackstash/encoder/message.rb

@@ -30,6 +30,7 @@ module Rackstash
     #     # Logs "[foo,123] [127.0.0.1] Hello\n[foo,123] [127.0.0.1] World\n"
     class Message
       include Rackstash::Encoder::Helper::Message
+      include Rackstash::Encoder::Helper::Timestamp
 
       # @param tagged [Array<#to_s>] An array of field names whose values are
       #   added in front of each message line on encode
@@ -46,7 +47,10 @@ module Rackstash
         message = event[FIELD_MESSAGE]
 
         unless message.empty?
-          tags = @tagged_fields.map { |key| format_tag event[key] }.compact.join
+          tags = @tagged_fields.map { |key|
+            normalize_timestamp(event, key)
+            format_tag event[key]
+          }.compact.join
           message = message.gsub(/^/) { tags } unless tags.empty?
         end
 

spec/rackstash/encoder/message_spec.rb

@@ -39,6 +39,18 @@ describe Rackstash::Encoder::Message do
         expect(encoder.encode(event)).to eql "[foo,bar] line1\n[foo,bar] line2\n"
       end
 
+      it 'normalizes the timestamp' do
+        time = Time.parse('2016-10-17 13:37:00 +03:00')
+        event = { 'message' => ["line1\n", "line2\n"], '@timestamp' => time }
+
+        tagged << '@timestamp'
+
+        expect(encoder.encode(event)).to eql [
+          "[2016-10-17T10:37:00.000000Z] line1\n",
+          "[2016-10-17T10:37:00.000000Z] line2\n"
+        ].join
+      end
+
       it 'ignores missing fields' do
         event = { 'message' => ["line1\n", "line2\n"] }
         expect(encoder.encode(event)).to eql "line1\nline2\n"