code: private static String extractTagSource(Map<String, String> tag_map) {
          return Optional.ofNullable(tag_map.remove("source"))
                  .orElseGet(() -> {
                      return Optional.ofNullable(tag_map.get("cluster"))
                              .map(WavefrontStrings::escapeTagValue)
                              .orElseGet(() -> tag_map.getOrDefault("moncluster", "monsoon"));
                  });
      }
docs: Extract the 'source' tag from the tag_map.
  
  Wavefront requires the 'source' tag to be the first tag on the line,
  hence the special handling. It also *must* be present, so we can never
  return null.