Take the speed radar sign from lesson 01 again. Counting cars by model gives a few dozen cells, known in advance; counting cars by license plate gives one cell per car that passes, without limit, and the results board becomes unreadable and costs a fortune to store. A Prometheus label is the column by which you count. route="/cours/{id}" is the model; route="/cours/C0042" is the plate. A metric's cardinality is the number of cells its labels open, and each cell is a time series Prometheus must keep in memory and on disk.
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