Tracing 
The Three Pillars of Observability: The Unification That Never Quite Arrived (Part 2 of 2)
Part 1 was about how the three pillars split apart. This part is about how many smart people, starting in 2018, tried to put them back together — what they actually built, and why none of it quite reached the finish line.
The Three Pillars of Observability: A History No One Planned (Part 1 of 2)
Today we treat metrics, logging, and tracing as the natural structure of observability. But it wasn’t designed; it grew. Part 1, on how it split apart (2010–2017).