@Override
    public boolean equals(Object o) {
        return o instanceof ConfigSubscription<?> cs && key.equals(cs.key);
    }
Correct type of ConfigSubscription instance based on type of source or form of config id

@param key        a {@link ConfigKey}
@return a subclass of a ConfigsSubscription
config/src/main/java/com/yahoo/config/subscription/impl/ConfigSubscription.java