wordpress-develop/tests/performance/wp-content/mu-plugins/server-timing.php

<?php

add_action(
	'template_redirect',
	static function() {

		global $timestart;

		$server_timing_values = array();
		$template_start       = microtime( true );

		$server_timing_values['before-template'] = $template_start - $timestart;

		ob_start();

		add_action(
			'shutdown',
			static function() use ( $server_timing_values, $template_start ) {

				global $timestart;

				$output = ob_get_clean();

				$server_timing_values['template'] = microtime( true ) - $template_start;

				$server_timing_values['total'] = $server_timing_values['before-template'] + $server_timing_values['template'];

				$header_values = array();
				foreach ( $server_timing_values as $slug => $value ) {
					if ( is_float( $value ) ) {
						$value = round( $value * 1000.0, 2 );
					}
					$header_values[] = sprintf( 'wp-%1$s;dur=%2$s', $slug, $value );
				}
				header( 'Server-Timing: ' . implode( ', ', $header_values ) );

				echo $output;
			},
			PHP_INT_MIN
		);
	},
	PHP_INT_MAX
);