What should I do, as a reviewer, if I see another reviewer suggesting the authors to cite papers not relevant to the manuscript? I have just submitted my review of a manuscript to a journal, which allowed me to see other reviewers' comments once I submitted mine. I have points of disagreement with this other reviewer, but I am sure this is normal. What alarmed me was that, his/her review comments ended with a suggestion for the authors to cite additional references which, in my opinion, are not directly relevant to the manuscript, but were suggested on the pretense that they were published in the same journal, which is weird. These references have something in common in them: they are authored by the same group of people. Although I cannot be sure that this reviewer is one of them, I have the feeling that h/she may be, and that this practice may be common. In the past, when I was the author, a reviewer also suggested me to include additional references, but these references were not authored by the same people, and anyway I ended up not including them as I thought they were not directly relevant. But as a reviewer, what should I do in this case?
I would recommend raising your concern to the editor. If the editor is honest, they will not ask the authors to follow the citations en masse.

Given that the citations are all in the same journal, however, it is possible that the editor is part of the same citation cartel and may blow off your concern. In this case, it may be worth raising the concern to the publisher and/or one of the major indices. Citation cartels have been gaining attention in recent years, and if this is part of a systematic pattern of coercive citation, then those larger organizations may be motivated to investigate and sanction.