public static String getReservedMetadataBlobId(Map<String, Object> args) throws RestServiceException {
    return getHeader(args, Headers.RESERVED_METADATA_ID, false);
  }
Return the reserved metadata id if set in the request args. Return {@code null} if not set in args.
@param args The request arguments.
@return the reserved metadata blob id if set in the request args. {@code null} otherwise.
@throws RestServiceException if exception happens while parsing args.
ambry-api/src/main/java/com/github/ambry/rest/RestUtils.java