[misc] mongodb: export a helper for adding new collections to db

Co-Authored-By: Eric Mc Sween <eric.mcsween@overleaf.com>
Co-Authored-By: Simon Detheridge <s@sd.ai>

services/docstore/app/js/mongodb.js

@@ -18,9 +18,16 @@ async function setupDb() {
   db.docs = internalDb.collection('docs')
   db.docOps = internalDb.collection('docOps')
 }
+async function addCollection(name) {
+  await waitForDb()
+  const internalDb = (await clientPromise).db()
+
+  db[name] = internalDb.collection(name)
+}
 
 module.exports = {
   db,
   ObjectId,
+  addCollection,
   waitForDb
 }