chore(slogutil): add configurable logging format (fixes #10352) (#10354)

This adds several options for configuring the log format of timestamps
and severity levels, making it more suitable for integration with log
systems like systemd.

      --log-format-timestamp="2006-01-02 15:04:05"
         Format for timestamp, set to empty to disable timestamps ($STLOGFORMATTIMESTAMP)

      --[no-]log-format-level-string
         Whether to include level string in log line ($STLOGFORMATLEVELSTRING)

      --[no-]log-format-level-syslog
         Whether to include level as syslog prefix in log line ($STLOGFORMATLEVELSYSLOG)

So, to get a timestamp suitable for systemd (syslog prefix, no level
string, no timestamp) we can pass `--log-format-timestamp=""
--no-log-format-level-string --log-format-level-syslog` or,
equivalently, set `STLOGFORMATTIMESTAMP="" STLOGFORMATLEVELSTRING=false
STLOGFORMATLEVELSYSLOG=true`.

Signed-off-by: Jakob Borg <jakob@kastelo.net>

internal/slogutil/leveler.go

@@ -9,6 +9,7 @@ package slogutil
 import (
 	"log/slog"
 	"maps"
+	"strings"
 	"sync"
 )
 
@@ -39,6 +40,24 @@ func SetDefaultLevel(level slog.Level) {
 	globalLevels.SetDefault(level)
 }
 
+func SetLevelOverrides(sttrace string) {
+	pkgs := strings.Split(sttrace, ",")
+	for _, pkg := range pkgs {
+		pkg = strings.TrimSpace(pkg)
+		if pkg == "" {
+			continue
+		}
+		level := slog.LevelDebug
+		if cutPkg, levelStr, ok := strings.Cut(pkg, ":"); ok {
+			pkg = cutPkg
+			if err := level.UnmarshalText([]byte(levelStr)); err != nil {
+				slog.Warn("Bad log level requested in STTRACE", slog.String("pkg", pkg), slog.String("level", levelStr), Error(err))
+			}
+		}
+		globalLevels.Set(pkg, level)
+	}
+}
+
 type levelTracker struct {
 	mut      sync.RWMutex
 	defLevel slog.Level