Merged PR 2730: Exposing Billable Bookings metric for KPIs

# Description

Exposes Billable Bookings metric for KPIs, both in the "global+dimension" view and in the "deal" view.

Metrics have already been created for a while. Exposing them now after the changes carried out in the model `int_core__booking_charge_events`. Based on the current quality of the data, I opted for "Est. Billable Bookings" to account for the fact that this is an estimation. If you don't feel comfortable with it, let's remove the "Est. ".

# Checklist

- [X] The edited models and dependants run properly with production data.
- [X] The edited models are sufficiently documented.
- [X] The edited models contain PK tests, and I've ran and passed them.
- [X] I have checked for DRY opportunities with other models and docs.
- [X] I've picked the right materialization for the affected models.

# Other

- [ ] Check if a full-refresh is required after this PR is merged.

Related work items: #18111

models/reporting/general/monthly_aggregated_metrics_history_by_deal.sql

@@ -13,6 +13,7 @@ select
     created_bookings as created_bookings,
     check_out_bookings as check_out_bookings,
     cancelled_bookings as cancelled_bookings,
+    billable_bookings as billable_bookings,
     created_guest_journeys as created_guest_journeys,
     started_guest_journeys as started_guest_journeys,
     completed_guest_journeys as completed_guest_journeys,