timetakenmiddleware.cs

  public async Task Invoke(HttpContext context)
        {
            var stopWatch = new Stopwatch();
            stopWatch.Start();

            context.Response.OnStarting(_ =>
                {
                    ((HttpContext)_).Response.Headers["X-Instrumentation-Time-Elapsed-Ms"] = stopWatch.ElapsedMilliseconds.ToString();
                    return Task.FromResult(0);
                }, context);

            stopWatch.Stop();
            
            await _next(context);
        }