docs: public static SpanId fromLowerBase16(CharSequence src) {
      Utils.checkNotNull(src, "src");
      // TODO: Remove this extra condition.
      Utils.checkArgument(
          src.length() == BASE16_SIZE,
          "Invalid size: expected %s, got %s",
          BASE16_SIZE,
          src.length());
      return fromLowerBase16(src, 0);
    }