Implementing A Nimbus Experiment

Follow instructions in https://experimenter.info/mobile-feature-api. Example implementation here

Nimbus FML https://experimenter.info/fml-spec/

There are some clarification on how to test your Nimbus implementation:

1. Add nimbus.remote-settings.url=https://settings-cdn.stage.mozaws.net to local.properties.
2. After building Fenix, make sure you turn on Secret Settings -> Use Nimbus Preview Collections.
3. The experiment in https://stage.experimenter.nonprod.dataops.mozgcp.net/nimbus/ does not have to be live for the test. In preview is sufficient.
4. Example of a test is here
5. Make sure you archive the test after you're done with it.
6. In your PR, make sure to submit the change for .experimenter.yaml as well.