Incorrect priority of the fluentd-apt-source package

Describe the bug

The priority of the fluentd-apt-source 2020.8.25-1 package is "important". It should be optional rather than important.

The deb package priority is defined in the debian policy document:

Most Debian packages will have a priority of optional. Priority levels other than optional are only used for packages that should be included by default in a standard installation of Debian.
https://www.debian.org/doc/debian-policy/ch-archive.html#priorities

$ dpkg -I /var/cache/apt/archives/fluentd-apt-source_2020.8.25-1_all.deb
 new Debian package, version 2.0.
 size 5340 bytes: control archive=752 bytes.
      40 bytes,     1 lines      conffiles
     442 bytes,    12 lines      control
     326 bytes,     4 lines      md5sums
 Package: fluentd-apt-source
 Version: 2020.8.25-1
 Architecture: all
 Maintainer: Fluentd developers <fluentd@googlegroups.com>
 Installed-Size: 22
 Depends: apt-transport-https, gnupg
 Section: misc
 Priority: important      # <------- It should be optional.
 Homepage: https://www.fluentd.org/
 Description: GnuPG archive key and APT source of the Treasure Agent archive
  The Treasure Agent project digitally signs its Release files. This
  package contains the archive key used for that.

To Reproduce

None

Expected behavior

The priority for the fluentd-apt-source package is optional.

Your Environment

$ cat /etc/os-release
PRETTY_NAME="Ubuntu 22.04.1 LTS"
NAME="Ubuntu"
VERSION_ID="22.04"
VERSION="22.04.1 LTS (Jammy Jellyfish)"
VERSION_CODENAME=jammy
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
UBUNTU_CODENAME=jammy

$ td-agent --version
td-agent 4.4.2 fluentd 1.15.3 (e89092ce1132a933c12bb23fe8c9323c07ca81f5)

$ dpkg -l 'fluent*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name               Version      Architecture Description
+++-==================-============-============-==============================================================
ii  fluentd-apt-source 2020.8.25-1  all          GnuPG archive key and APT source of the Treasure Agent archive

# /etc/apt/sources.list.d/fluentd.sources
Types: deb
URIs: https://packages.treasuredata.com/4/ubuntu/jammy/
Suites: jammy
Components: contrib
Signed-By: /usr/share/keyrings/fluentd-archive-keyring.gpg


### Your Configuration

```apache
None

Your Error Log

None

Additional context

When we trying to install high priority packages by tasksel install standard, the fluentd-apt-source package installed unintentionally. It breaks apt sources.list because it conflicts between manually added settings /etc/apt/sources.d/packages_treasuredata_com_4_ubuntu_jammy.list and package added settings /etc/apt/sources.list.d/fluentd.sources.

The reason of the fluentd-apt-source package installed unintentionally, it has high priority.

fluent / fluent-package-builder