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 #208922 (closed)

# cat /var/log/gitlab/sidekiq-cluster/current | grep Starting
{"severity":"INFO","time":"2020-03-04T14:13:26.463Z","message":"Starting cluster with 2 processes"}

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)