Move JsonLogger to CE and add spec

spec/lib/gitlab/json_logger_spec.rb

+# coding: utf-8
+require 'spec_helper'
+
+describe Gitlab::JsonLogger do
+  subject { described_class.new('/dev/null') }
+
+  let(:now) { Time.now }
+
+  describe '#format_message' do
+    it 'formats strings' do
+      output = subject.format_message('INFO', now, 'test', 'Hello world')
+      data = JSON.parse(output)
+
+      expect(data['severity']).to eq('INFO')
+      expect(data['time']).to eq(now.utc.iso8601(3))
+      expect(data['message']).to eq('Hello world')
+    end
+
+    it 'formats hashes' do
+      output = subject.format_message('INFO', now, 'test', { hello: 1 })
+      data = JSON.parse(output)
+
+      expect(data['severity']).to eq('INFO')
+      expect(data['time']).to eq(now.utc.iso8601(3))
+      expect(data['hello']).to eq(1)
+      expect(data['message']).to be_nil
+    end
+  end
+end