AC

• Confirm that IDs generated by the Metrics Platform client should be UUIDs
  • The Android app's Event Platform client generates 20-character hex strings: https://github.com/wikimedia/apps-android-wikipedia/blob/65bf40da3f79823e4419cd3df[…]va/org/wikipedia/analytics/eventplatform/EventPlatformClient.kt
• [Maybe] Copy the algorithm above into the 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 1 below)

Notes

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)

We can use https://commons.apache.org/proper/commons-math/javadocs/api-3.6.1/org/apache/commons/math3/stat/inference/KolmogorovSmirnovTest.html in the Java implementation