Enables Shared Storage callers to customize the number of contributions per Private Aggregation report. This feature enables Shared Storage callers to configure per-context contribution limits via a new field, `maxContributions`. Callers set this field to override the default number of contributions per report — larger and smaller numbers will both be permitted. Chrome will accept values of `maxContributions` between 1 and 1000 inclusive; larger values will be interpreted as 1000. Due to padding, the size of each report's payload will be roughly proportional to the chosen number of contributions per report. We expect that opting into larger reports will increase the cost of operating the Aggregation Service. Protected Audience callers will not be affected by this feature. However, we are planning to add support for customizing the number of contributions for Protected Audience reports in future features.
Private Aggregation imposes a limit on the number of histogram contributions that can be embedded in a single aggregatable report, with any additional contributions being dropped. Shared Storage callers can technically work around the limit by invoking another Shared Storage operation, but it's not ideal. Some callers may even want fewer than 20 contributions per report, but for privacy reasons all reports are padded up to the fixed size. Enabling callers to fine-tune the number of contributions per report will (1) enable those that want more contributions per report to more ergonomically make use of their allotted privacy budget and (2) reduce processing costs on the Aggregation Service for those that want fewer contributions.