HttpApiGuidlinesMiddleware.cs

class HttpApiGuidlinesMiddleware : OwinMiddleware
{
    public HttpApiGuidlinesMiddleware(OwinMiddleware next) : base(next) { }

    public override async Task Invoke(IOwinContext context)
    {
        var isApiCall = context.Request.Path.StartsWithSegments(new PathString("/api"));

        Stream originalStream = null;
        Stream responseBuffer = null;

        if (isApiCall)
        {
            originalStream = context.Response.Body;
            responseBuffer = new MemoryStream();
            context.Response.Body = responseBuffer;

            context.Response.Headers.Append("x-request-id", Guid.NewGuid().ToString("N"));
        }
        await Next.Invoke(context);

        if (isApiCall)
        {
            responseBuffer.Seek(0, SeekOrigin.Begin);
            var responseReader = new StreamReader(responseBuffer);
            var response = await responseReader.ReadToEndAsync();

            using (MD5 md5 = MD5.Create())
            {
                byte[] hasedBytes = md5.ComputeHash(Encoding.UTF8.GetBytes(response));
                var result = new StringBuilder(hasedBytes.Length * 2);

                for (int i = 0; i < hasedBytes.Length; i++)
                {
                    result.Append(hasedBytes[i].ToString("x2"));
                }

                context.Response.Headers.Append("Etag", result.ToString());

                responseBuffer.Seek(0, SeekOrigin.Begin);
                responseBuffer.CopyTo(originalStream);
            }
        }
    }
}