Per SERVER-91218, we check this flag multiple times from the same thread, and this risks reading different values. In the case of this feature, the impact of that problem is fairly low - i.e., we might request metrics for some parts of a query and not others, and therefore undercount certain metrics for any queries that are sampled for query stats during an unstable FCV period. It shouldn't cause a mongos to request metrics from a mongod that doesn't support them, which would be more serious.