static NumberFormatException isntLowerHexLong(String lowerHex) {
    throw new NumberFormatException(
        lowerHex + " should be a 1 to 32 character lower-hex string with no prefix");
  }
Parses a 16 character lower-hex string with no prefix into an unsigned long, starting at the
specified index.
zipkin/src/main/java/zipkin2/internal/HexCodec.java