public int size() {
        return userConfigsMap.size();
    }
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