Output job statistics in bg migration

db/post_migrate/20210825193652_backfill_candence_id_for_boards_scoped_to_iteration.rb

@@ -37,7 +37,11 @@ def schedule_backfill_project_boards
       delay = index * DELAY
       migrate_in(delay, MIGRATION, ['project', 'up', *range])
+
+      increment_job_counter!
     end
+
+    report_and_reset_job_counter!
   end
   def schedule_backfill_group_boards
@@ -46,6 +50,30 @@ def schedule_backfill_group_boards
       delay = index * DELAY
       migrate_in(delay, MIGRATION, ['group', 'up', *range])
+
+      increment_job_counter!
     end
+
+    report_and_reset_job_counter!
+  end
+
+  def increment_job_counter!
+    @job_counter = (job_counter + 1)
+  end
+
+  def job_counter
+    @job_counter ||= 0
+  end
+
+  def report_and_reset_job_counter!
+    duration = DELAY * job_counter
+
+    say <<~SAY
+      Scheduled #{job_counter} #{MIGRATION} jobs with an interval of #{DELAY} seconds.
+
+      The migration is expected to take at least #{duration} seconds. Expect all jobs to have completed after #{Time.zone.now + duration}."
+    SAY
+
+    @job_counter = 0
   end
 end