private Mono<Void> dropAllDatabases() {
        Mono<Void> dropDatabase = Mono.empty();
        for (TurmsMongoClient client : clients) {
            dropDatabase = dropDatabase.then(Mono.defer(client::dropDatabase));
        }
        return dropDatabase;
    }
@return True if all collections have existed
turms-service/src/main/java/im/turms/service/storage/mongo/MongoCollectionInitializer.java