`postinst` hook ignores `policy-rc.d` on Debian-alikes

[DWSR]

Output of the info page (if this is a bug)
N/A

Describe what happened:

• I updated /usr/sbin/policy-rc.d to exit with code 101 always, which should cause services installed via APT to not start automatically.
• I installed the datadog-agent package via APT.
• The datadog-agent service was automatically enabled and started, but crashed because configuration was incomplete.

Describe what you expected:
I expected the datadog-agent package to honour policy-rc.d and not start the service

The postinst hook should use deb-systemd-invoke and debian-systemd-helper from init-system-helpers, rather than invoking systemctl directly:

https://manpages.debian.org/testing/init-system-helpers/deb-systemd-helper.1p.en.html
https://manpages.ubuntu.com/manpages/focal/man1/deb-systemd-invoke.1p.html

Steps to reproduce the issue:
See above.

Additional environment details (Operating System, Cloud provider, etc):
Ubuntu 20.04 LTS, but this should be reproducible on any Debian-alike.

[NikitaCOEUR]

This issue is getting old, but the problem is still with us .... Datadog is quite expensive .... We would like to have a better control of our costs....

On our infrastructure, datadog-agent is deployed and maintained with puppet. But given the exorbitant cost of hosts, we don't necessarily want our entire infrastructure to be running datadog at the same time. We have several environments on which we'd like to activate datadog for a given period of time.
Also, we've provided a means of activating the agent on demand for a specified period on a specified host. This works well. But every time datadog-agent is updated by puppet, all datadog services are executed, causing all our hosts to be run on datadog (and costs to skyrocket) even though we don't use them.

There are methods, via policy-rd or via systemd presets, to not allow packages to run services normally ... but it doesn't work here, probably because of the postinst scripts on debian :
https://github.com/DataDog/datadog-agent/blob/main/omnibus/package-scripts/agent-deb/postinst#L149-L186