@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        for (Map.Entry<ConfigKey<?>, ConfigInstance.Builder> entry : configs.entrySet()) {
            sb.append(entry.getKey()).append("=>").append(entry.getValue());
        }
        return sb.toString();
    }
Returns true if this set contains a config instance matching the given key

@param key a config key
@return a ConfigInstance
config/src/main/java/com/yahoo/config/subscription/ConfigSet.java