Create Experiment Metric

Defines a metric once, then reference its id from any number of experiments. type is binomial (did the visitor convert) or mean (how much), numerator sets the aggregation plus event filters (e.g. name = "signup"), and window bounds how long after exposure a conversion counts. No data is posted here — the tracker keeps collecting events as usual; this is only the recipe the snapshot compute reads. Validation happens in the ingest service.

Use it when you need to measure something no existing metric covers. Metrics are reusable and role-free — an experiment decides whether a given metric is its goal or a guardrail.