Track created_at on CI minutes records

Add `created_at` to `ci_project_monthly_usages` and
`ci_namespace_monthly_usages` to track exactly when
monthly records are created lazily.

Changelog: added

db/migrate/20211115145107_add_created_at_to_namespace_monthly_usages.rb

+# frozen_string_literal: true
+
+class AddCreatedAtToNamespaceMonthlyUsages < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      add_column :ci_namespace_monthly_usages, :created_at, :datetime_with_timezone
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :ci_namespace_monthly_usages, :created_at
+    end
+  end
+end

db/migrate/20211115154103_add_created_at_to_project_monthly_usage.rb

+# frozen_string_literal: true
+
+class AddCreatedAtToProjectMonthlyUsage < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      add_column :ci_project_monthly_usages, :created_at, :datetime_with_timezone
+    end
+  end
+
+  def down
+    with_lock_retries do
+      remove_column :ci_project_monthly_usages, :created_at
+    end
+  end
+end

db/schema_migrations/20211115145107

+2b6bc8067402744b79eee06022cf3c91ba7ffd519df83aae4067600a6bbf43ad
\ No newline at end of file

db/schema_migrations/20211115154103

+ad65e6deb885397dc91f33dc117a50e9a1b6d60f4caed8c5b77d474ec0340995
\ No newline at end of file

db/structure.sql

@@ -11748,6 +11748,7 @@ CREATE TABLE ci_namespace_monthly_usages (
     amount_used numeric(18,2) DEFAULT 0.0 NOT NULL,
     notification_level smallint DEFAULT 100 NOT NULL,
     shared_runners_duration integer DEFAULT 0 NOT NULL,
+    created_at timestamp with time zone,
     CONSTRAINT ci_namespace_monthly_usages_year_month_constraint CHECK ((date = date_trunc('month'::text, (date)::timestamp with time zone)))
 );
 
@@ -11993,6 +11994,7 @@ CREATE TABLE ci_project_monthly_usages (
     date date NOT NULL,
     amount_used numeric(18,2) DEFAULT 0.0 NOT NULL,
     shared_runners_duration integer DEFAULT 0 NOT NULL,
+    created_at timestamp with time zone,
     CONSTRAINT ci_project_monthly_usages_year_month_constraint CHECK ((date = date_trunc('month'::text, (date)::timestamp with time zone)))
 );
 

ee/spec/models/ci/minutes/namespace_monthly_usage_spec.rb

@@ -40,6 +40,7 @@ RSpec.describe Ci::Minutes::NamespaceMonthlyUsage do
           expect(subject.namespace).to eq(namespace)
           expect(subject.date).to eq(described_class.beginning_of_month)
           expect(subject.notification_level).to eq(::Ci::Minutes::Notification::PERCENTAGES.fetch(:not_set))
+          expect(subject.created_at).to eq(Time.current)
         end
       end
     end

ee/spec/models/ci/minutes/project_monthly_usage_spec.rb

@@ -32,6 +32,7 @@ RSpec.describe Ci::Minutes::ProjectMonthlyUsage do
           expect(subject.amount_used).to eq(0)
           expect(subject.project).to eq(project)
           expect(subject.date).to eq(described_class.beginning_of_month)
+          expect(subject.created_at).to eq(Time.current)
         end
       end
     end