p99, not the average
An average of 80 milliseconds can hide that one request in a hundred takes six seconds, and those are the requests people complain about.
Graph p50, p95 and p99 on the same panel. Set the alert on p99. The average is useful for capacity maths and for nothing else. Histograms in the metrics library give you all three for free. A gauge of the mean gives you none of them.
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