agent: Fix a bug where all syslog lines are notice when using JSON

[jrasell]

The agent syslog write handler was unable to handle JSON log lines correctly, meaning all syslog entries when using JSON log format showed as NOTICE level.

This change adds a new handler to the Nomad agent which can parse JSON log lines and correctly understand the expected log level entry.

The change also removes the use of a filter from the default log format handler. This is not needed as the logs are fed into the syslog handler via hclog, which is responsible for level filtering.

Testing & Reproduction steps

Running on a Linux workstation (Ubuntu 24.04) I added the following lines to the /etc/rsyslog.conf before restarting rsyslog via sudo service rsyslog restart.

$template verbose, "%syslogseverity-text%, %syslogfacility%, %timegenerated%, %HOSTNAME%, %syslogtag%, %msg%\n"
*.* /var/log/all-messages.log;verbose

I then ran Nomad via sudo -i nomad agent -dev -log-json -log-level=<LEVEL> -config=/home/jrasell/config.hcl using the following configuration:

enable_syslog = true

Links

Jira: https://hashicorp.atlassian.net/browse/NET-11973

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[tgross]

LGTM