Bring JsonLogger to CE

This backports a module that was in EE for Geo so that other modules
can have structured logging support.

lib/gitlab/json_logger.rb

+module Gitlab
+  class JsonLogger < ::Gitlab::Logger
+    def self.file_name_noext
+      raise NotImplementedError
+    end
+
+    def format_message(severity, timestamp, progname, message)
+      data = {}
+      data[:severity] = severity
+      data[:time] = timestamp.utc.iso8601(3)
+
+      case message
+      when String
+        data[:message] = message
+      when Hash
+        data.merge!(message)
+      end
+
+      data.to_json + "\n"
+    end
+  end
+end

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