Mongo explain results incorrectly returning an object instead of an array of objects

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

Mongo explain results incorrectly returning an object instead of an array of objects. This causes an issue for SDKs that use strongly typed languages as results (plural) should always be an array.

This will break any SDK that isn't strongly typed. In the case of the JS SDK, it will break current testcases instead (PR for JS SDK here. In addition, this brings up another issue in the JS SDK which doesn't support explain for first queries.

Originally discussed on community.parseplatform.org

Steps to reproduce

Run any explain query.

Actual Outcome

A single explain object.

Expected Outcome

An array of explain objects that have only 1 object.

Failing Test Case / Pull Request

• 🤩 I submitted a PR with a fix and a test case.
• 🧐 I submitted a PR with a failing test case.

Environment

Server

• Parse Server version: 4.5.0 and latest master
• Operating system: docker linux
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): self hosted

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 4.4
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): self hosted

Client

• SDK (iOS, Android, JavaScript, PHP, Unity, etc): Swift SDK
• SDK version: 1.8.3

Logs

Start by reproducing any MongoDB explain query in Parse Server and trace the response handling; the issue names no source file or test. Use the submitted failing-test and fix PR as context, and consider the issue complete when the response exposes results as an array containing the single explain object.