Gil Tene (Azul Systems) has argued convincingly [1] (slides [2]) that monitoring tools get latency measurement wrong because sudden spikes aren't represented correctly in timings because of averaging and the wrong use of percentiles.
He argues that percentiles simply aren't useful, because, statistically, most requests will experience >= 99.99-percentile response times. All percentiles lie; the only truly useful and realistic number is, in fact, the 100th percentile.
He also argues that the most revealing way to show latency is with a complete histogram.
I haven't used K6 yet, but I've been looking for a good load-testing tool, and intend to try it out.
[1] https://www.youtube.com/watch?v=lJ8ydIuPFeU
[2] https://www.azul.com/files/HowNotToMeasureLatency_LLSummit_N...