journald log driver does not support priority prefixes

[tmds]

I have the following dotnet-demo.container unit:

[Unit]
Description=Example .NET systemd service

[Service]
TimeoutStartSec=900
Restart=always

[Container]
Image=quay.io/tmds/systemd-demo-service:latest
AutoUpdate=registry
Notify=true
LogDriver=journald

[Install]
WantedBy=default.target

The application writes messages to standard output that are prefixed with a journal priority.

The journald logdriver doesn't understand this priority, and the priority appears literally in the log message instead:

{
...
	"PRIORITY" : "6",
...
	"MESSAGE" : "<7>Microsoft.Extensions.Hosting.Internal.Host[2] Hosting started\n",

---

Comment from @Luap99 (copied from #27038 (comment)):

Mhh looks like that format is described here https://www.freedesktop.org/software/systemd/man/latest/sd-daemon.html

So I guess it would make sense if the journald driver understands that format as well and parse the priority for it.
cc @jnovy @mheon

But that is something that would need to happen in conmon