I'm mentioning this because I'm getting the impression that it is not common knowledge. Apologies if that is the wrong impression.
It is a real structural loss to drill holes in a round tube. I know, nearly everyone does it. This is a case where getting away with it doesn't mean that it's right or the correct thing to do. Eventually those holes will cause a failure of the tube. How soon depends on the severity of the use, the size of the hole, the size of the tube, and on the weight attached & it's leverage.
If a tube must have a hole in it then it should have a sleave completely through the hole and be welded on both sides.
Much easier to weld on tabs instead.