Currently, libmongocrypt simply chooses a random value if a FLE2 field has a contention factor max greater than zero. It would be nice if we can give the user more control over which contention factor value to use, and it'll also help us write predictable CRUD unit tests in the server once we switch to using the libmongocrypt API completely. One possible way would be to allow users to register a callback function to the mongocrypt_ctx_t that libmongocrypt can call instead of _mongocrypt_random_int64().