Fix for count being very slow on large Parse Classes' collections (#5264)

* * Added fix for MongoCollection's count function, so that it uses the much more effecient estimatedDocumentCount if no queries were specified

* * Added missing options when running estimatedDocumentCount for Mongo Collections

* * Fixed issue with checking for zero query for Mongo Collections count

src/Adapters/Storage/Mongo/MongoCollection.js

@@ -80,6 +80,16 @@ export default class MongoCollection {
   }
 
   count(query, { skip, limit, sort, maxTimeMS, readPreference } = {}) {
+    // If query is empty, then use estimatedDocumentCount instead.
+    // This is due to countDocuments performing a scan,
+    // which greatly increases execution time when being run on large collections.
+    // See https://github.com/Automattic/mongoose/issues/6713 for more info regarding this problem.
+    if (typeof query !== 'object' || !Object.keys(query).length) {
+      return this._mongoCollection.estimatedDocumentCount({
+        maxTimeMS,
+      });
+    }
+
     const countOperation = this._mongoCollection.countDocuments(query, {
       skip,
       limit,