Improve patching of Redis interceptor

Prepend the class in an initializer so that it does not depend on
autoloading

config/initializers/7_redis.rb

 # frozen_string_literal: true
 
+Redis::Client.prepend(Gitlab::Instrumentation::RedisInterceptor)
+
 # Make sure we initialize a Redis connection pool before multi-threaded
 # execution starts by
 # 1. Sidekiq

lib/gitlab/instrumentation/redis_interceptor.rb

 # frozen_string_literal: true
 
-require 'redis'
-
 module Gitlab
   module Instrumentation
     module RedisInterceptor
@@ -99,7 +97,3 @@ module Gitlab
     end
   end
 end
-
-class ::Redis::Client
-  prepend ::Gitlab::Instrumentation::RedisInterceptor
-end