Cancel experiment

Cancel an experiment, ending it without a winning variant. The experiment moves to CANCELLED status, the supplied reason is recorded in its conclusion as the decision reason, and the experiment is unlinked from the feature flag allocations that exposed it, which stops its exposure. An experiment that has already completed its rollout, had its code removed, or been canceled cannot be canceled again. Canceling is not reversible: an experiment cannot be returned to a running state afterward. It is also not idempotent: canceling an already-canceled experiment returns 409, so a retry after a timeout cannot be distinguished from a cancellation made by someone else.