Add `usage_start_time` and `usage_end_time` to GCP billing SQL queries

Billing export data ingested shows incorrect daily costs because the Lens visualization relies on `@timestamp`, which reflects ingestion time rather than when the cost was incurred. As a result, historical costs appear on the ingestion date instead of their actual usage period.

We need to update the SQL queries to include `usage_start_time` / `usage_end_time` and adjust the visualization to use those fields as the time axis.