wp-config.php

<?php

global $wp_filter;

if ( ! $wp_filter ) {
	$wp_filter = array(
		'parse_request' => array(),
		'rest_post_dispatch' => array(),
	);
}

// set the newrelic values to the matched query
$wp_filter['parse_request'][0] = array(
	'newrelic' => array(
		'function' => function( WP $wp ) {
			if ( extension_loaded( 'newrelic' ) ) {
				newrelic_name_transaction( $wp->matched_rule );
			}
		},
		'accepted_args' => 1,
	),
);

$wp_filter['rest_post_dispatch'][0] = array(
	'newrelic' => array(
		'function' => function( $result ) {
			if ( extension_loaded( 'newrelic' ) ) {
				newrelic_name_transaction( '/wp-json' . $result->get_matched_route() );
			}
			return $result;
		},
		'accepted_args' => 1,
	),
);