Actually i solved it with a class model container which contains my model object and the additional information i need as properties (those will be automatically added to my Json):

public abstract class JsonBaseFormatter
{
     public bool Success { get; set; } = true;
     public int Duration { get; set; }
}

public class JsonFormatter<T> : JsonBaseFormatter
{
     public T Data { get; set; }

     public JsonFormatter(T data)
     {
        Data = data;
     }
}

Then in my controller i used this class container in the HttpGet retreive method:

[HttpGet("{cd_drt_cml}")]
[ProducesResponseType(typeof(Ds_Invit_Sydrtcml), StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status404NotFound)]
public ActionResult<JsonFormatter<Ds_Invit_Sydrtcml>> RetrieveOne(string cd_drt_cml)
{
        var model = _ids_invit_sydrtcmlservice.RetrieveOne(cd_drt_cml);

        if (model == null)
        {
             return NotFound();
        }

        var response = new JsonFormatter<Ds_Invit_Sydrtcml>(model);
            
        return new ActionResult<JsonFormatter<Ds_Invit_Sydrtcml>>(response);

}

And then the beauty of it, I created an ActionFilter that is called automatically when http requests are sent or received by the controller:

 public class ResponseTimeActionFilter : IActionFilter
 {
     private const string ResponseTimeKey = "ResponseTimeKey";
     public void OnActionExecuting(ActionExecutingContext context)
     {
         // Start the timer   
         context.HttpContext.Items[ResponseTimeKey] = Stopwatch.StartNew();
     }

     public void OnActionExecuted(ActionExecutedContext context)
     {
         var res = context.Result as ObjectResult;
         if (res != null)
         {
             Stopwatch stopwatch = (Stopwatch)context.HttpContext.Items[ResponseTimeKey];
             // Calculate the time elapsed   
             var timeElapsed = stopwatch.Elapsed;
             var success = context.HttpContext.Response.StatusCode == StatusCodes.Status200OK;
                
             var json = ((ObjectResult)context.Result).Value as JsonBaseFormatter;
             if (json != null)
             {                   
                 json.Success = success;
                 json.Duration = timeElapsed.Milliseconds;
             }
          }
      }
 }

To make the filter works you have to register it in the Startup->ConfigureService method:  services.AddScoped<ResponseTimeActionFilter>();

And also decorate your controller so that it automatically calls the filter at each http request operation:  [ServiceFilter(typeof(ResponseTimeActionFilter))]

all infos about how filters work here https://damienbod.com/2015/09/15/asp-net-5-action-filters/ 

The request result can be found in attachment.