Add Logstash encoder

This encoder is very similar to the JSON encoder. It handles a few
additional specifics for Logstash to be suotable for Logstash's json
input.

lib/rackstash/encoders/logstash.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'rackstash/encoders/json'
+
+module Rackstash
+  module Encoders
+    # The JSON encoder formats the log event as a single-line JSON string. The
+    # resulting JSON string contains all data exposed by the buffer. Leading
+    # and trailing whitespace as well as any ANSI color codes in the `"message"`
+    # field will be removed.
+    #
+    # The resulting string is in the JSON format native to Logstash. You can
+    # thus ship your logs directly to Logstash without further processing by
+    # using Logstash's [json codec](https://www.elastic.co/guide/en/logstash/current/plugins-codecs-json.html)
+    # on the input definition.
+    class Logstash < JSON
+      # @param event [Hash] a log event as produced by the {Flow}
+      # @return [String] the event as a single-line JSON string
+      def encode(event)
+        event[FIELD_VERSION] = '1'.freeze if event[FIELD_VERSION].nil?
+        super(event)
+      end
+    end
+  end
+end

spec/rackstash/encoders/logstash_spec.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+# Copyright 2017 Holger Just
+#
+# This software may be modified and distributed under the terms
+# of the MIT license. See the LICENSE.txt file for details.
+
+require 'spec_helper'
+
+require 'rackstash/encoders/logstash'
+
+describe Rackstash::Encoders::Logstash do
+  let(:encoder) { described_class.new }
+
+  describe '#encode' do
+    it 'formats the passed event hash as a JSON string and includes @version' do
+      event = { 'hello' => 'world', 'message' => 'hello' }
+      expect(encoder.encode(event)).to eql '{"hello":"world","message":"hello","@version":"1"}'
+    end
+  end
+end