Concurrent request middleware (only works on ASP.NET because I'm lazy). Also does not support websocket requests.

ConcurrentRequestViewer.cs

private class ConcurrentRequestViewer
{
    private readonly Func<IDictionary<string, object>, Task> _next;
    private static ConcurrentDictionary<string, Task> _requests = new ConcurrentDictionary<string, Task>();

    public ConcurrentRequestViewer(Func<IDictionary<string, object>, Task> next)
    {
        _next = next;
    }

    public Task Invoke(IDictionary<string, object> env)
    {
        var path = (string)env["owin.RequestPath"];

        if (path.EndsWith("/requests"))
        {
            var writer = new StreamWriter((Stream)env["owin.ResponseBody"]);
            using (writer)
            {
                writer.Write("<pre>");
                foreach (var req in _requests)
                {
                    if (!req.Value.IsCompleted)
                    {
                        writer.WriteLine(req.Key);
                    }
                }
                writer.Write("</pre>");
            }
            var tcs = new TaskCompletionSource<object>();
            tcs.SetResult(null);
            return tcs.Task;
        }

        var context = (HttpContextBase)env[typeof(HttpContextBase).FullName];

        string url = context.Request.RawUrl;
        Task task = _next(env);

        _requests.TryAdd(url, task);

        task.ContinueWith(_ =>
        {
            // Remove the task when it's complete
            Task t;
            _requests.TryRemove(url, out t);
        });

        return task;
    }
}