ahmedimraan

This plugin adds Custom Fields to WP REST API JSON output. Works with Advanced Custom Fields. 
Its as simple as Installing and Activating, once activated you can see an object named metadata.

WP REST API POST/ PAGE CUSTOM FIELDS

Adds Custom Field output to the WP REST API for posts, pages, users, and taxonomies.

Martin von Berg

Extend the REST-API to work with WordPress Media-Library. Organize images in Folders. Add and Update images including Metadata and Posts using the images. Access with Authorization only. 
This plugin extends the REST-API of WordPress to directly access the Media-Library for Images. It is intended to be used together with a Lightroom Plugin or as a stand-alone interface for headless WordPress. The new REST-API endpoints (functions) allow to add additional metadata to images, update existing metadata or update images completely without changing the WordPress-ID. Images may be added to the standard directory hierarchy of wordpress or to an additional folder which allows better organization and searching for images. 
NEW FUNCTION : See 3.2

Media Library Extension

Extend the REST-API to work with Wordpress Media-Library. Organize images in Folders. Add and Update images including Metadata and Posts using the ima …

WP REST API POST/ PAGE CUSTOM FIELDS vs Media Library Extension