fluentd: gem built in ubuntu 18.04 cannot be installed for td-agent 0.12.43

$ td-agent --version
td-agent 0.12.43

hearen@hearen-pc:/opt/fluent-plugin-prometheus$ uname -r
4.15.0-46-generic

hearen@hearen-pc:/opt/fluent-plugin-prometheus$ cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.2 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.2 LTS"
VERSION_ID="18.04"
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"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

I created my own plugin for prometheus collection and tried to install the td-agent following these two options in 18.04:

option - 1:

curl -L https://toolbelt.treasuredata.com/sh/install-ubuntu-bionic-td-agent3.sh | sh

option - 2: download its deb: https://td-agent-package-browser.herokuapp.com/2.5/ubuntu/bionic/pool/contrib/t/td-agent

My configuration looks

<source>
  @type prometheus
  port 7081
</source>

<source>
  @type prometheus_monitor
</source>

<source>
  @type prometheus_output_monitor
</source>

<source>
  @type prometheus_tail_monitor
</source>

My Error is that I cannot build a valid gem in 18.04 which means I can build a valid gem in 16.04

td-agent-gem build ./fluent-plugin-prometheus.gemspec -q

If I built the gem in 18.04, td-agent then cannot be restarted and print error log as

2019-04-01 18:16:52 +0800 [info]: adding match pattern="debug.**" type="stdout"
2019-04-01 18:16:52 +0800 [info]: adding source type="prometheus"
2019-04-01 18:16:52 +0800 [error]: config error file="/etc/td-agent/td-agent.conf" error="Unknown input plugin 'prometheus'. Run 'gem search -rd fluent-plugin' to find plugins"
2019-04-01 18:16:52 +0800 [info]: process finished code=256
2019-04-01 18:16:52 +0800 [warn]: process died within 1 second. exit.

I’m quite new in fluentd/td-agent, perhaps it’s silly question. Please provide any clue you have to help.

Any help will be appreciated 😉