faster metadata serialization

    std::string s;

    // The order is important. don't change.

    s += fmt::format_int(mode_).c_str(); // add mandatory mode

    s += dentry_val_delim + fmt::format_int(size_).c_str(); // add mandatory size

    s += dentry_val_delim;

    s += fmt::format_int(size_).c_str(); // add mandatory size

    if (ADAFS_DATA->atime_state()) {

        s += dentry_val_delim + fmt::format_int(atime_).c_str();

        s += dentry_val_delim;

        s += fmt::format_int(atime_).c_str();

    }

    if (ADAFS_DATA->mtime_state()) {

        s += dentry_val_delim + fmt::format_int(mtime_).c_str();

        s += dentry_val_delim;

        s += fmt::format_int(mtime_).c_str();

    }

    if (ADAFS_DATA->ctime_state()) {

        s += dentry_val_delim + fmt::format_int(ctime_).c_str();

        s += dentry_val_delim;

        s += fmt::format_int(ctime_).c_str();

    }

    if (ADAFS_DATA->uid_state()) {

        s += dentry_val_delim + fmt::format_int(uid_).str();

        s += dentry_val_delim;

        s += fmt::format_int(uid_).c_str();

    }

    if (ADAFS_DATA->gid_state()) {

        s += dentry_val_delim + fmt::format_int(gid_).str();

        s += dentry_val_delim;

        s += fmt::format_int(gid_).c_str();

    }

    if (ADAFS_DATA->inode_no_state()) {

        s += dentry_val_delim + fmt::format_int(inode_no_).str();

        s += dentry_val_delim;

        s += fmt::format_int(inode_no_).c_str();

    }

    if (ADAFS_DATA->link_cnt_state()) {

        s += dentry_val_delim + fmt::format_int(link_count_).c_str();

        s += dentry_val_delim;

        s += fmt::format_int(link_count_).c_str();

    }

    if (ADAFS_DATA->blocks_state()) {

        s += dentry_val_delim + fmt::format_int(blocks_).c_str();

        s += dentry_val_delim;

        s += fmt::format_int(blocks_).c_str();

    }

    return s;

}