Ensure sidekiq-cluster emits JSON logs

We can just re-use the Sidekiq JSON formatter to ensure this happens
since sidekiq-cli doesn't use the Sidekiq initializer.

Part of https://gitlab.com/gitlab-org/gitlab/issues/208922

ee/lib/gitlab/sidekiq_cluster/cli.rb

@@ -3,6 +3,7 @@
 require 'optparse'
 require 'logger'
 require 'time'
+require_relative '../../../../lib/gitlab/sidekiq_logging/json_formatter'
 
 module Gitlab
   module SidekiqCluster
@@ -24,13 +25,9 @@ module Gitlab
         @alive = true
         @processes = []
         @logger = Logger.new(log_output)
+        @logger.formatter = ::Gitlab::SidekiqLogging::JSONFormatter.new
         @rails_path = Dir.pwd
         @dryrun = false
-
-        # Use a log format similar to Sidekiq to make parsing/grepping easier.
-        @logger.formatter = proc do |level, date, program, message|
-          "#{date.utc.iso8601(3)} #{Process.pid} TID-#{Thread.current.object_id.to_s(36)} #{level}: #{message}\n"
-        end
       end
 
       def run(argv = ARGV)

lib/gitlab/sidekiq_logging/json_formatter.rb

 # frozen_string_literal: true
 
+# This is needed for sidekiq-cluster
+require 'json'
+
 module Gitlab
   module SidekiqLogging
     class JSONFormatter