code: static final Set<String> uniquePrefixSet(final Config config) {
  
        Set<String> prefixSet = Sets.newHashSet();
        for(Map.Entry<String, ConfigValue> entry : config.entrySet()) {
           String key = entry.getKey();
           int index = key.indexOf('.');
           if(index > 0) {
              String prefix = key.substring(0, index);
              prefixSet.add(prefix);
           }
        }
  
        return prefixSet;
     }
docs: Gets all available keys in the config.
  @param config The config.
  @return The keys.