fix specs, refactor missing bits from events stuff

lib/gitlab/cycle_analytics/base_event_fetcher.rb

lib/gitlab/cycle_analytics/base_stage.rb

lib/gitlab/cycle_analytics/event.rb

lib/gitlab/cycle_analytics/metrics_fetcher.rb

spec/lib/gitlab/cycle_analytics/code_event_fetcher_spec.rb

spec/lib/gitlab/cycle_analytics/issue_event_fetcher_spec.rb

spec/lib/gitlab/cycle_analytics/plan_event_fetcher_spec.rb

...b/gitlab/cycle_analytics/production_event_fetcher_spec.rb

spec/lib/gitlab/cycle_analytics/review_event_fetcher_spec.rb

spec/lib/gitlab/cycle_analytics/shared_event_spec.rb

spec/lib/gitlab/cycle_analytics/shared_stage_spec.rb

.../lib/gitlab/cycle_analytics/staging_event_fetcher_spec.rb

spec/lib/gitlab/cycle_analytics/test_event_fetcher_spec.rb