You start towards it in your post, but don't explicitly call it out: the central task is decreasing complexity. A "dumb" heartbeat message, and a check for its

You start towards it in your post, but don't explicitly call it out: the central task is decreasing complexity. A "dumb" heartbeat message, and a check for its absence, is reliable precisely because of its simplicity. So if the question is "How do I monitor my complicated observability stack?" then the answer, to me, starts with "What is the simplest method of accomplishing the minimal version of my goals?"