static int decodeLowerHex(char c) {
      if (c >= '0' && c <= '9') return c - '0';
      if (c >= 'a' && c <= 'f') return c - 'a' + 10;
      throw new AssertionError("not lowerHex " + c); // bug
    }
@param length is greater than zero
zipkin/src/main/java/zipkin2/internal/Proto3Fields.java