logging: Add journald encoder wrapper

[steadytao]

Implements the #7611 feature request as an explicit logging encoder wrapper.

Summary

This adds a new journald encoder that wraps another encoder and prepends the systemd/journald priority prefix (<N>) to each emitted log line.

Example:

log {
    format journald {
        wrap json
    }
}

Why this shape

The issue discussion converged on keeping this:

• opt-in rather than environment-detected
• at the encoder layer rather than the writer/output layer
• as a wrapper around an existing encoder rather than a completely separate transport

That means the wrapped encoder still controls the actual log format (json, console, etc.), while journald only adds the priority prefix that journald understands for stdout/stderr log streams.

This encoder does not write directly to journald.

Behaviour

Priority mapping currently follows the standard journald/syslog severity levels:

• debug -> <7>
• info -> <6>
• warn -> <4>
• error -> <3>
• dpanic / panic / fatal -> <2>

If no wrapped encoder is specified, it follows the same default behavior as the existing wrapper encoders:

• default to json
• switch to console for terminal output

Tests

Added:

• unit tests for severity prefix mapping and encoded output
• a Caddyfile adaptation test for format journald { wrap console }

EDIT

Updated to also suppress the wrapped encoder’s timestamp by default when using format journald.

That means:

• wrap console no longer includes the in-line timestamp
• wrap json no longer includes the ts field

The intent is to avoid duplicating timestamp information that journald / journalctl already provides separately. Should be mentioned explicitly.

Implements + should close #7611.

Assistance Disclosure

No AI was used.

[steffenbusch]

I tested it with both json and console

        log journal_log {
                output stderr
                format journald {
                        wrap console
                }
        }

It looks much better in journalctl now:

Can it be configured so that the timestamp from the log message is dropped to remove that redundancy, as it is already present in journalctl?

[steadytao]

My plan will be to make the journald wrapper suppress the wrapped encoder’s timestamp by default, so:

• wrap console no longer includes the in-line timestamp
• wrap json no longer includes the ts field

I’ll take a look at the cleanest way to do that without making the wrapper too invasive.

[steadytao]

The journald wrapper now suppresses the timestamp emitted by the wrapped encoder by default, so:

• wrap console drops the in-line timestamp
• wrap json drops the ts field

That keeps journald’s own timestamp as the one shown by journalctl instead of duplicating it inside the log payload. (expected behaviour when toggled to unify with how journald is expected to be used)