@SuppressWarnings({"unchecked", "rawtypes"})
    public void addBuilder(String configId, ConfigInstance.Builder builder) {
        Class<?> configClass = builder.getClass().getDeclaringClass();
        ConfigKey<?> key = new ConfigKey(configClass, configId);
        configs.put(key, builder);
    }
Inserts a new builder in this set. If an existing entry exists, it is overwritten.

@param configId The config id for this builder.
@param builder The builder that will produce config for the particular config id.
config/src/main/java/com/yahoo/config/subscription/ConfigSet.java