Language and Product Localization team is planning to conduct an experiment using Experimentation Lab to test MinT for Readers as a hypothesis in FY25/26 WE 3.1

Their data collection requires translation schema fragment and the analytics/product_metrics/web/translation schema (related: T396900: Update analytics/product_metrics/web/translation to use latest common fragment (1.4.2)). They can't use product_metrics.web_base stream, so they need a way of instrumenting their experiment and send() events to a custom stream.

Acceptance Criteria:

• Provide a method to override the schema ID and the stream name in Experiment.js
• Update documentation and unit tests
  • https://wikitech.wikimedia.org/wiki/Metrics_Platform/Conduct_an_experiment#Client-side_instrumentation