I have a 15 gallon kettle and am looking to add a sight gauge to it. I have been looking around and have come across a couple that I like. I would rather get a sight gauge that is encased in metal like the one here. However, I have not read in the description of this gauge or any like it (with the encasing) that include some kind of calibration labels. The only sight gauge I can find that include the labels is here. Am I missing something? Or are some manufacturers not including the labels?
The problem with sight gauges with pre-defined volume labels is that the manufacturer is not able to determine what size kettle you are attaching it to, and even if you are an 1/4 of an inch off, you could possibly be a 1/4 - 1/2 gallon off the mark (depending on how wide the kettle is). I guarantee the 5-gallon mark on my Blichmann boil kettle is not going to be the same on any other kettle.
So far as I can tell, it looks as though the labels in the second link you provided come separate, and you have to apply them yourself. To do that you'll need to install the sight glass as instructed, and then fill the kettle gallon by gallon (or liter by liter), and then mark them off on the glass as it's filled up. You can get the nice vinyl labels, or I've seen a lot of people settle for permanent marker.
That said, I would highly encourage the most robust sight glass you can buy. Blichmann puts a metal shield (with the readings engraved) on either side of the glass, the first one you linked seems like it would be a good viable option as well. Just remember, if that thing shatters, you now have a hole in your kettle for wort to come pouring out of.