@Override
    public String toString() {
        return "StoreLabel{" + "key='" + key + '\'' + ", value='" + value + '\'' + '}';
    }
Case insensitive key/value for replica constraints.

@author jiachun.fjc
jraft-rheakv/rheakv-core/src/main/java/com/alipay/sofa/jraft/rhea/metadata/StoreLabel.java