Track step duration with monotonic time

Because it never jumps, monotonic time is more reliable for creating this data set

lib/gitlab/ci/pipeline/chain/sequence.rb

@@ -14,12 +14,15 @@ module Gitlab
 
           def build!
             @sequence.each do |step_class|
-              step_start = Time.now
+              step_start = ::Gitlab::Metrics::System.monotonic_time
               step = step_class.new(@pipeline, @command)
 
               step.perform!
 
-              @command.observe_step_duration(step_class, Time.now - step_start)
+              @command.observe_step_duration(
+                step_class,
+                ::Gitlab::Metrics::System.monotonic_time - step_start
+              )
 
               break if step.break?
             end