Issue Details

Caddy simply writes to stdout/stderr by default. This is perfectly fine, but makes it rather cumbersome to filter events of a certain level using journalctl later on.

The good thing is that systemd interprets a custom level prefix by default: https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#SyslogLevelPrefix=

Here you can find the list of levels: https://www.freedesktop.org/software/systemd/man/latest/sd-daemon.html

#define SD_EMERG   "<0>"  /* system is unusable */
#define SD_ALERT   "<1>"  /* action must be taken immediately */
#define SD_CRIT    "<2>"  /* critical conditions */
#define SD_ERR     "<3>"  /* error conditions */
#define SD_WARNING "<4>"  /* warning conditions */
#define SD_NOTICE  "<5>"  /* normal but significant condition */
#define SD_INFO    "<6>"  /* informational */
#define SD_DEBUG   "<7>"  /* debug-level messages */

e.g this line would be logged as a warning to journald with the <4> prefix getting stripped:

<4>{"level":"warn","ts":1775048791.656852,"logger":"http","msg":"HTTP/2 skipped because it requires TLS","network":"tcp","addr":":80"}

It would be nice if either caddy would output the prefix by default if executed via systemd or it could be enabled per configuration.

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