addServerTiming is a node function that wraps a sync or async function and adds server-timing headers with the elapsed time.

addServerTiming.ts

/**
  * @example
  * const users = addServerTiming(responseHeaders, 'LU', () => db.fetch('users'))
  * @see https://web.dev/custom-metrics/?utm_source=devtools#server-timing-api
  */
export default function addServerTiming<T>(headers: Headers, label: string, callback: () => T): T {
  const start = Date.now();
  const maybePromise = callback();

  if (maybePromise instanceof Promise) {
    return maybePromise.then((result) => {
      const end = Date.now();
      headers.set(
        "Server-Timing",
        [headers.get("Server-Timing"), `${label};dur=${end - start}`].filter(Boolean).join(", "),
      );
      // eslint-disable-next-line @typescript-eslint/no-unsafe-return
      return result;
    }) as T;
  }

  const end = Date.now();
  headers.set(
    "Server-Timing",
    [headers.get("Server-Timing"), `${label};dur=${end - start}`].filter(Boolean).join(", "),
  );
  return maybePromise;
}