Add load tests to CI

[alexeyr-ci]

#453 adds K6 scripts for load testing the dummy app pages, but they currently need to be run manually. Add them to CI. See https://grafana.com/blog/2022/03/06/load-testing-with-circleci/ for guidance.

What needs to be decided:

• Run locally on CircleCI machines, or using a free Grafana Cloud plan?
• When to run? Definitely on merges into main, but probably we don't want them on every push, since running either locally or in the cloud uses some CircleCI credits. Maybe make them triggerable but require a success before merge, if that's possible.
• Visualization https://grafana.com/blog/2023/04/11/how-to-visualize-load-testing-results/
• Detecting regressions: https://grafana.com/docs/grafana-cloud/testing/k6/analyze-results/test-comparison/, Compare the results of two different test runs grafana/k6-jslib-summary#2
  • We could also just add thresholds on metrics, but they would be a bit arbitrary and inconvenient to maintain, the solutions above should be better.