Populate image title, alt-text, caption, and description on upload (only for images uploaded via XML-RPC)

functions.php

<?php
// Do not include the opening php tag.

// Populate image title, alt-text, caption, and description on upload
// but only if upload is via XML-RPC (thanks Colin Walker)
add_action ('xmlrpc_call', 'check_new_attachment' );

function check_new_attachment( $method ) {
	if( 'metaWeblog.newMediaObject' === $method ) {
		add_action( 'add_attachment', 'crt_set_image_meta' );
	}
}

function crt_set_image_meta( $post_ID ) {

	// Check if uploaded file is an image, else do nothing
	if ( wp_attachment_is_image( $post_ID ) ) {

		// Set image meta to 'Status: Date'
		$timezone = get_option('timezone_string');
		date_default_timezone_set( $timezone );
		$image_title = 'Status: '.date( 'Y-m-d H.i.s' );
		$image_meta = array(

			'ID'		=> $post_ID,
			'post_title'	=> $image_title,
			'post_excerpt'	=> $image_title,
			'post_content'	=> $image_title,

		);

		// Set the image alt-text
		update_post_meta( $post_ID, '_wp_attachment_image_alt', $image_title );

		// Set the image meta
		wp_update_post( $image_meta );

	}

}

Updates the original code to only set the $image_meta fields to date/time for images uploaded by XML-RPC, e.g. the metadataless images uploaded by Micro.blog client apps; browser-based uploads will now retain the default WordPress behavior of setting $image_meta fields based on any IPTC metadata in the image.

Props to @colin-walker for finding and introducing me to the "is this happening via XML-RPC?" check and syntax.