Add user meta to the WP-API

wp-api-user-meta.php

<?php
/* Adds all user meta to the /wp-json/wp/v2/user/[id] endpoint */
function sb_user_meta( $data, $field_name, $request ) {
	if( $data['id'] ){
		$user_meta = get_user_meta( $data['id'] );
	}

	if ( !$user_meta ) {
    return new WP_Error( 'No user meta found', 'No user meta found', array( 'status' => 404 ) );
  }

  foreach ($user_meta as $key => $value) {
		$data[$key] = $value;
	}

	return $data;
}

add_action( 'rest_api_init', function () {
    register_api_field( 'user',
        'meta',
        array(
            'get_callback'    => 'sb_user_meta',
            'update_callback' => null, // add callback here for POST/PUT requests to update user meta
            'schema'          => null,
        )
    );
} );

Works great thanks @scottopolis . I added it in the functions.php file as is and testing GET /wp-json/wp/v2/users/2 in Postman returns all the meta fields.

How will I update the user meta?

@Wohlfath great ! Could you provide your functions.php please ?