refactor fetcher and fixed specs

...controllers/projects/cycle_analytics/events_controller.rb

lib/gitlab/cycle_analytics/base_event.rb

lib/gitlab/cycle_analytics/events_query.rb

lib/gitlab/cycle_analytics/metrics_fetcher.rb

lib/gitlab/database/median.rb

spec/models/cycle_analytics/code_spec.rb

spec/models/cycle_analytics/issue_spec.rb

spec/models/cycle_analytics/plan_spec.rb

spec/models/cycle_analytics/production_spec.rb

spec/models/cycle_analytics/review_spec.rb

spec/models/cycle_analytics/staging_spec.rb

spec/models/cycle_analytics/test_spec.rb

spec/support/cycle_analytics_helpers/test_generation.rb