Utilize scalar selectables to obtain stats

Instead of executing a selectable in a function. Returning a
scalar selectable allow the stats to be obtained through execution
of a single query containing many subqueries

datalad_registry/blueprints/api/dataset_urls/tools.py

@@ -200,12 +200,15 @@ def get_non_annex_ds_collection_stats(base_cte: CTE) -> ScalarSelect:
         .subquery("non_annex_ds_q")
     )
 
-    return select(
-        func.jsonb_build_object(
-            "ds_count",
-            select(func.count()).select_from(non_annex_ds_q).scalar_subquery(),
-        ).label("non_annex_ds_collection_stats")
-    ).scalar_subquery()
+    return (
+        select(
+            func.jsonb_build_object("ds_count", func.count()).label(
+                "non_annex_ds_collection_stats"
+            )
+        )
+        .select_from(non_annex_ds_q)
+        .scalar_subquery()
+    )
 
 
 def get_collection_stats(select_stmt: Select) -> CollectionStats: