Add method for getting just the report count of an aggregate share

crates/daphne-service-utils/src/durable_requests/bindings/aggregate_store.rs

@@ -25,6 +25,7 @@ super::define_do_binding! {
         Merge = "/internal/do/aggregate_store/merge",
         MarkCollected = "/internal/do/aggregate_store/mark_collected",
         CheckCollected = "/internal/do/aggregate_store/check_collected",
+        ReportCount = "/internal/do/aggregate_store/report_count"
     }
 
     fn name((version, task_id, bucket): (DapVersion, &'n TaskId, &'n DapBatchBucket)) -> ObjectIdFrom {

crates/daphne-worker/src/durable/aggregate_store.rs

@@ -234,6 +234,16 @@ impl AggregateStore {
         Ok(())
     }
 
+    async fn agg_share_report_count(&self) -> Result<u64> {
+        if let Some(agg_share) = &self.agg_share {
+            return Ok(agg_share.report_count);
+        };
+        Ok(self
+            .get::<DapAggregateShareMetadata>(METADATA_KEY)
+            .await?
+            .map_or(0, |m| m.report_count))
+    }
+
     async fn get_agg_share(&mut self) -> Result<&mut DapAggregateShare> {
         let keys = Self::agg_share_shard_keys();
         let agg_share = if let Some(agg_share) = self.agg_share.take() {
@@ -477,6 +487,15 @@ impl GcDurableObject for AggregateStore {
                 Response::from_json(&agg_share)
             }
 
+            // Get the number of reports in this aggregate share.
+            //
+            // Idempotent
+            // Output: `u64`
+            Some(bindings::AggregateStore::ReportCount) => {
+                let count = self.agg_share_report_count().await?;
+                Response::from_json(&count)
+            }
+
             // Mark this bucket as collected.
             //
             // Non-idempotent (do not retry)