Added Server Timing output for Ajax and REST APIs.

Fixes #13

src/Manager/Server_Timing_Manager.php

@@ -0,0 +1,71 @@
+<?php
+declare( strict_types=1 );
+
+namespace Rarst\Laps\Manager;
+
+use Rarst\Laps\Plugin;
+use Rarst\Laps\Record\Record_Interface;
+
+/**
+ * Implements Server Timing API headers.
+ */
+class Server_Timing_Manager {
+
+	/** @var Plugin */
+	private $laps;
+
+	/**
+	 * @param Plugin $laps Container instance.
+	 */
+	public function __construct( Plugin $laps ) {
+
+		$this->laps = $laps;
+
+		add_action( 'admin_init', [ $this, 'send_timing_header' ], PHP_INT_MAX );
+		add_action( 'rest_pre_serve_request', [ $this, 'send_timing_header' ] );
+	}
+
+	/**
+	 * Sends Server Timing API header with records data.
+	 *
+	 * @param mixed $input Pass-through input for attaching to a filter hook.
+	 *
+	 * @return mixed
+	 */
+	public function send_timing_header( $input ) {
+
+		if ( headers_sent() ) {
+			return $input;
+		}
+
+		$ajax = defined( 'DOING_AJAX' ) && DOING_AJAX;
+		$rest = defined( 'REST_REQUEST' ) && REST_REQUEST;
+
+		if ( ! $ajax && ! $rest ) {
+			return $input;
+		}
+
+		// This will be false in REST API w/o nonce, even with valid cookie. Might make sense to filter to true in dev.
+		if ( ! apply_filters( 'laps_can_see', current_user_can( 'manage_options' ) ) ) {
+			return $input;
+		}
+
+		$records = $this->laps['records'];
+		$header  = '';
+
+		/** @var Record_Interface $record */
+		foreach ( $records as $record ) {
+			$duration = $record->get_duration() * 1000;
+
+			if ( $duration < 1 ) {
+				continue;
+			}
+
+			$header .= sprintf( '%s;dur=%d;desc="%s", ', $record->get_category(), $duration, $record->get_name() );
+		}
+
+		header( 'Server-Timing: ' . $header );
+
+		return $input;
+	}
+}