As a follow up to T375900: Allow users to override experiment enrollment, we should prove uniform distribution of user enrollment assignments in experiments managed by Metrics Platform.

Description

Add another test to the UserSplitterInstrumentation class in the Metrics Platform extension by injecting randomness into the user ID value (user hash) and checking that resultant variant values have set membership instead of evaluating to a deterministic value when there are multiple variant values.

See https://gitlab.wikimedia.org/phuedx/id-distribution-test as an example of an ID distribution test.

Acceptance Criteria

• Relevant test is added and passes

Required

• Unit/Integration tests?
• Documentation?
• Passed QA?