Failed to install plugin with td-agent v3

[pikatenor]

fluent-plugin-google-cloud gem fails to install, with latest oldstable td-agent v3.8.1.

Steps to reproduce:

 $ docker run -it ubuntu:bionic
 # curl -L https://toolbelt.treasuredata.com/sh/install-ubuntu-bionic-td-agent3.sh | sh
 # td-agent-gem install fluent-plugin-google-cloud

Log:

 $ docker run --rm -it ubuntu:bionic
root@885654dc6db7:/# apt update && apt install -y curl sudo gnupg2
(omitted... 129 added, 0 removed; done.)
root@885654dc6db7:/# curl -L https://toolbelt.treasuredata.com/sh/install-ubuntu-bionic-td-agent3.sh | sh
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   851  100   851    0     0   6753      0 --:--:-- --:--:-- --:--:--  6700
==============================
 td-agent Installation Script 
==============================
This script requires superuser access to install apt packages.
You will be prompted for your password by sudo.
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  3157  100  3157    0     0  33585      0 --:--:-- --:--:-- --:--:-- 33585
OK
Get:1 http://packages.treasuredata.com/3/ubuntu/bionic bionic InRelease [7692 B]
Get:2 http://packages.treasuredata.com/3/ubuntu/bionic bionic/contrib amd64 Packages [2133 B]                                           
Hit:3 http://security.ubuntu.com/ubuntu bionic-security InRelease                                          
Hit:4 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:5 http://archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:6 http://archive.ubuntu.com/ubuntu bionic-backports InRelease
Fetched 9825 B in 1s (9892 B/s)
Reading package lists... Done
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following NEW packages will be installed:
  td-agent
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 56.0 MB of archives.
After this operation, 168 MB of additional disk space will be used.
Get:1 http://packages.treasuredata.com/3/ubuntu/bionic bionic/contrib amd64 td-agent amd64 3.8.1-0 [56.0 MB]
Fetched 56.0 MB in 1s (72.8 MB/s)   
debconf: delaying package configuration, since apt-utils is not installed
Selecting previously unselected package td-agent.
(Reading database ... 4847 files and directories currently installed.)
Preparing to unpack .../td-agent_3.8.1-0_amd64.deb ...
Unpacking td-agent (3.8.1-0) ...
Setting up td-agent (3.8.1-0) ...
Adding system user `td-agent' (UID 101) ...
Adding new group `td-agent' (GID 101) ...
Adding new user `td-agent' (UID 101) with group `td-agent' ...
Creating home directory `/var/lib/td-agent' ...
Installing default conffile /etc/td-agent/td-agent.conf ...
invoke-rc.d: could not determine current runlevel
invoke-rc.d: policy-rc.d denied execution of start.
Processing triggers for libc-bin (2.27-3ubuntu1.4) ...

Installation completed. Happy Logging!

root@885654dc6db7:/# td-agent-gem install fluent-plugin-google-cloud
Fetching: fluentd-1.11.2.gem (100%)
Successfully installed fluentd-1.11.2
Fetching: google-protobuf-3.15.8-x86_64-linux.gem (100%)
Successfully installed google-protobuf-3.15.8-x86_64-linux
Fetching: googleapis-common-protos-types-1.1.0.gem (100%)
Successfully installed googleapis-common-protos-types-1.1.0
Fetching: grpc-1.31.1-x86_64-linux.gem (100%)
Successfully installed grpc-1.31.1-x86_64-linux
Fetching: googleapis-common-protos-1.3.10.gem (100%)
Successfully installed googleapis-common-protos-1.3.10
Fetching: faraday-0.17.4.gem (100%)
Successfully installed faraday-0.17.4
Fetching: jwt-2.2.3.gem (100%)
Successfully installed jwt-2.2.3
Fetching: memoist-0.16.2.gem (100%)
Successfully installed memoist-0.16.2
Fetching: os-1.1.1.gem (100%)
Successfully installed os-1.1.1
Fetching: signet-0.15.0.gem (100%)
ERROR:  Error installing fluent-plugin-google-cloud:
        signet requires Ruby version >= 2.5.

It seems that google-api-client is directly depends on signet, but several other dependencies have dropped their support for Ruby 2.4.

googleapis/signet#178
googleapis/google-api-ruby-client#2841
googleapis/google-auth-library-ruby#300

Will this gem also no longer support for Ruby 2.4 / td-agent v3?