An online advertising platform uses Cloud Spanner for ad impression tracking. The table 'ad_impressions' has a primary key (ad_id, timestamp). The table receives millions of writes per minute. A secondary index on (campaign_id, timestamp) was created to support queries that sum impressions per campaign. During high traffic, the team notices increased write latency and hotspotting on the index (the campaign_id has low cardinality, causing all writes to a campaign to hit the same index split). They need to redesign the schema to avoid hotspotting on the index while still supporting the campaign aggregation queries. What is the best solution?