public int addLearners(final Iterable<PeerId> learners) {
        int ret = 0;
        if (learners != null) {
            for (final PeerId peer : learners) {
                if (this.learners.add(peer.copy())) {
                    ret++;
                }
            }
        }
        return ret;
    }
Add learners in batch, returns the added count.

@param learners learners to add
@return the total added count
jraft-core/src/main/java/com/alipay/sofa/jraft/conf/Configuration.java