Allow generator CLI to generate new definitions using the key 'tiers'

scripts/internal_events/cli/metric.rb

scripts/internal_events/cli/metric_definer.rb

...pts/internal_events/metrics/ee_total_28d_single_event.yml

...ipts/internal_events/metrics/ee_total_7d_single_event.yml

...scripts/internal_events/metrics/ee_total_single_event.yml

...s/internal_events/metrics/keyboard_smashed_metric_28d.yml

...ts/internal_events/metrics/keyboard_smashed_metric_7d.yml

...vents/metrics/label_28d_single_event_additional_props.yml

...events/metrics/label_7d_single_event_additional_props.yml

...nternal_events/metrics/project_id_28d_multiple_events.yml

...internal_events/metrics/project_id_7d_multiple_events.yml

...vents/metrics/total_multi_event_some_additional_props.yml

...rnal_events/metrics/total_multiple_events_with_rename.yml

...es/scripts/internal_events/metrics/total_single_event.yml

...ipts/internal_events/metrics/user_id_28d_single_event.yml

...nts/metrics/user_id_28d_single_event_additional_props.yml

...metrics/user_id_28d_single_event_all_additional_props.yml

...ripts/internal_events/metrics/user_id_7d_single_event.yml

...ents/metrics/user_id_7d_single_event_additional_props.yml

.../metrics/user_id_7d_single_event_all_additional_props.yml