AC

• Copy the algorithm in mw.user.generateRandomSessionId into the JS client library to generate pageview/session IDs
• A unit test is added to test the distribution of pageview/session IDs generated by the algorithm (see note 2 below)

See https://gitlab.wikimedia.org/phuedx/id-distribution-test.

Notes

1. The algorithm would be best placed in DefaultIntegration (or nearby?) so that it's testable in isolation and so that DefaultIntegration#getPageviewID() and #getSessionID() get implementations!

In T304379#9400845, @mpopov wrote:

• We don't actually need to test if ~10% of sessions are determined to be in sample when sample rate is 0.1, etc. – INSTEAD we just need to verify that the distribution of those N numbers (obtained from "normalizing" the randomly generated session IDs) is approximately Uniform(0,1).
  • Fortunately there's a statistical test for that – the Kolmogorov–Smirnov test. Actually, pages 139-140 of https://www.johndcook.com/Beautiful_Testing_ch10.pdf describe the procedure and suggests thresholds to use.
  • We can even use an existing library to accomplish this: https://www.npmjs.com/package/@stdlib/stats-kstest
    • PASS if pValue >= 0.001 (fail to reject the null hypothesis that the two distributions are the same)
    • FAIL if pValue < 0.001 (reject the null in favor of the alternative hypothesis that the two distributions are different)