refactored metrics fetcher - merged into stage and events

lib/gitlab/cycle_analytics/base_event_fetcher.rb

lib/gitlab/cycle_analytics/base_query.rb

lib/gitlab/cycle_analytics/base_stage.rb

lib/gitlab/cycle_analytics/code_stage.rb

lib/gitlab/cycle_analytics/issue_stage.rb

lib/gitlab/cycle_analytics/metrics_fetcher.rb

lib/gitlab/cycle_analytics/plan_event_fetcher.rb

lib/gitlab/cycle_analytics/plan_stage.rb

lib/gitlab/cycle_analytics/production_helper.rb

lib/gitlab/cycle_analytics/production_stage.rb

lib/gitlab/cycle_analytics/review_stage.rb

lib/gitlab/cycle_analytics/staging_event_fetcher.rb

lib/gitlab/cycle_analytics/staging_stage.rb

lib/gitlab/cycle_analytics/summary/commit.rb

lib/gitlab/cycle_analytics/test_stage.rb

spec/lib/gitlab/cycle_analytics/shared_event_spec.rb