Background

In T404058: Add MetricsPlatform to gated extensions, I decided not to make MetricsPlatform a gated extension but instead to adopt the Stable Interface Policy for the xLab SDKs. This task covers doing that.

AC

• We agree on which parts form the stable interface for the xLab SDKs
  • e.g. ExperimentManager#getExperiment(): Experiment
• The stability of the PHP xLab SDK is documented
• The stability of the JS xLab SDK is documented