This quote from Charity Majors is probably the best summary of the current state of observability in the tech industry - a total, mass confusion. Everyone is confused. What is a trace? What is a span? Is log line a span? Do I need traces if I have logs? Why I need traces if I have great metrics?
R. Baeza-Yates, C. Hurtado, and M. Mendoza. Current Trends in Database Technology - EDBT 2004 Workshops, volume 3268 of Lecture Notes in Computer Science, Springer Berlin Heidelberg, (2005)