ProgressListenerPool pool = new ProgressListenerPool();

Retrofit retrofit = new Retrofit.Builder()
    .client(new OkHttpClient.Builder().addInterceptor(new ProgressInterceptor(pool)).build())
    .addConverterFactory(new ProgressConverterFactory(pool))
    ...
    .build();

public interface WS {

   @GET("/sth")
   Observable<Object> getSomething(@DownloadProgress @Header(DownloadProgress.HEADER) ProgressListener listener);

   @POST("/sth")
   Observable<Object> setSomething(@Body String veryLongString, @UploadProgress @Header(UploadProgress.HEADER) ProgressListener listener);
}