Make `Chaos.cpu_spin` to consume thread

This changes `.cpu_spin` to actually
consume thread for the desired duration,
by looking at CPU thread time.

lib/gitlab/chaos.rb

@@ -19,9 +19,11 @@ module Gitlab
 
     # cpu_spin will consume all CPU on a single core for the specified duration
     def self.cpu_spin(duration_s)
-      expected_end_time = Time.now + duration_s
+      return unless Gitlab::Metrics::System.thread_cpu_time
+
+      expected_end_time = Gitlab::Metrics::System.thread_cpu_time + duration_s
 
-      rand while Time.now < expected_end_time
+      rand while Gitlab::Metrics::System.thread_cpu_time < expected_end_time
     end
 
     # db_spin will query the database in a tight loop for the specified duration