@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        for (ConfigDefinitionKey key : userConfigsMap.keySet()) {
            sb.append(key.toString());
        }
        return sb.toString();
    }
A UserConfigRepo is a repository for user configs, typically for a particular config producer. The repo encapsulates
how the user configs are stored, and defines the methods to retrieve user configs and merge the repo with others.

@author Ulf Lilleengen
config-model/src/main/java/com/yahoo/config/model/producer/UserConfigRepo.java