public delegate Task RequestDelegate(HttpContext context);

public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware);

public static IApplicationBuilder UseMiddleware<TMiddleware>(this IApplicationBuilder app, params object[] args);

public class ResponseTimeMiddleware
{
  // Name of the Response Header, Custom Headers starts with "X-" 
  private const string RESPONSE_HEADER_RESPONSE_TIME = "X-Response-Time-ms";
  // Handle to the next Middleware in the pipeline 
  private readonly RequestDelegate _next;
  public ResponseTimeMiddleware(RequestDelegate next)
  {
    _next = next;
  }
  public Task InvokeAsync(HttpContext context)
  {
    // Start the Timer using Stopwatch 
    var watch = new Stopwatch();
    watch.Start();
    context.Response.OnStarting(() => {
      // Stop the timer information and calculate the time  
      watch.Stop();
      var responseTimeForCompleteRequest = watch.ElapsedMilliseconds;
      // Add the Response time information in the Response headers.  
      context.Response.Headers[RESPONSE_HEADER_RESPONSE_TIME] = responseTimeForCompleteRequest.ToString();
      return Task.CompletedTask;
    });
    // Call the next delegate/middleware in the pipeline  
    return this._next(context);
  }
}