aspnetcore/samples/Http2SampleApp/TimingMiddleware.cs

using System.Diagnostics;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;

namespace Http2SampleApp
{
    // You may need to install the Microsoft.AspNetCore.Http.Abstractions package into your project
    public class TimingMiddleware
    {
        private readonly RequestDelegate _next;

        public TimingMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public async Task Invoke(HttpContext httpContext)
        {
            if (httpContext.Response.SupportsTrailers())
            {
                httpContext.Response.DeclareTrailer("Server-Timing");

                var stopWatch = new Stopwatch();
                stopWatch.Start();

                await _next(httpContext);

                stopWatch.Stop();
                // Not yet supported in any browser dev tools
                httpContext.Response.AppendTrailer("Server-Timing", $"app;dur={stopWatch.ElapsedMilliseconds}.0");
            }
            else
            {
                // Works in chrome
                // httpContext.Response.Headers.Append("Server-Timing", $"app;dur=25.0");
                await _next(httpContext);
            }
        }
    }

    // Extension method used to add the middleware to the HTTP request pipeline.
    public static class TimingMiddlewareExtensions
    {
        public static IApplicationBuilder UseTimingMiddleware(this IApplicationBuilder builder)
        {
            return builder.UseMiddleware<TimingMiddleware>();
        }
    }
}