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/sloginit.go

@@ -10,20 +10,26 @@ import (
 	"io"
 	"log/slog"
 	"os"
-	"strings"
+	"time"
 )
 
 var (
-	GlobalRecorder = &lineRecorder{level: -1000}
-	ErrorRecorder  = &lineRecorder{level: slog.LevelError}
-	globalLevels   = &levelTracker{
+	GlobalRecorder    = &lineRecorder{level: -1000}
+	ErrorRecorder     = &lineRecorder{level: slog.LevelError}
+	DefaultLineFormat = LineFormat{
+		TimestampFormat: time.DateTime,
+		LevelString:     true,
+	}
+	globalLevels = &levelTracker{
 		levels: make(map[string]slog.Level),
 		descrs: make(map[string]string),
 	}
-	slogDef = slog.New(&formattingHandler{
-		recs: []*lineRecorder{GlobalRecorder, ErrorRecorder},
-		out:  logWriter(),
-	})
+	globalFormatter = &formattingOptions{
+		LineFormat: DefaultLineFormat,
+		recs:       []*lineRecorder{GlobalRecorder, ErrorRecorder},
+		out:        logWriter(),
+	}
+	slogDef = slog.New(&formattingHandler{opts: globalFormatter})
 )
 
 func logWriter() io.Writer {
@@ -38,21 +44,4 @@ func logWriter() io.Writer {
 
 func init() {
 	slog.SetDefault(slogDef)
-
-	// Handle legacy STTRACE var
-	pkgs := strings.Split(os.Getenv("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)
-	}
 }