public final FluentFuture<Void> ensure() {
      return repository.doIndex(fields, options);
    }
Creates configured index on a set of fields, if one does not already exist.
@see MongoCollection#createIndex(Bson, IndexOptions)
@return future of indexing operation, future value is insignificant ({@code null} typed as
        {@link Void})
mongo/src/org/immutables/mongo/repository/Repositories.java