Track shared runners duration when job completes

What does this MR do and why?

Related to #254231 (closed)

In this MR we introduce a FF ci_always_track_shared_runners_usage that enables tracking of shared_runners_duration when a job completes.

Today when a job completes we track the amount of CI minutes consumed as job.duration * runner cost_factor. Until this MR we only tracked amount_used but for old public projects we currently use a cost_factor: 0 which means that we don't count CI minutes consumption in this scenario. However, we would like to know how long the namespace used shared runners for, regardless of the CI minutes billing. This is why we added tracking of shared_runners_duration separately from CI minutes even when CI minutes consumption is 0.

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ee/app/models/ci/minutes/namespace_monthly_usage.rb

@@ -27,13 +27,14 @@ def self.find_or_create_current(namespace_id:)
         current_month.safe_find_or_create_by(namespace_id: namespace_id)
       end
 
-      def self.increase_usage(usage, amount)
-        return unless amount > 0
+      def self.increase_usage(usage, increments)
+        increment_params = increments.select { |_attribute, value| value > 0 }
+        return if increment_params.empty?
 
         # The use of `update_counters` ensures we do a SQL update rather than
         # incrementing the counter for the object in memory and then save it.
         # This is better for concurrent updates.
-        update_counters(usage, amount_used: amount)
+        update_counters(usage, increment_params)
       end
 
       def self.reset_current_usage(namespace)