feat(otel): add OpenTelemetry support

pkg/gotenberg/internal/log/stdhandler.go

@@ -0,0 +1,128 @@
+package log
+
+import (
+	"context"
+	"log/slog"
+	"os"
+	"strings"
+
+	"go.opentelemetry.io/otel/trace"
+	"golang.org/x/term"
+)
+
+const (
+	autoLoggingFormat = "auto"
+	jsonLoggingFormat = "json"
+	textLoggingFormat = "text"
+)
+
+// traceContextHandler is an internal wrapper that specifically injects
+// trace_id and span_id into the log record before they are written to the standard output.
+type traceContextHandler struct {
+	slog.Handler
+}
+
+func (h traceContextHandler) Handle(ctx context.Context, r slog.Record) error {
+	if spanCtx := trace.SpanContextFromContext(ctx); spanCtx.IsValid() {
+		// Since slog.Record is immutable with regards to adding attributes in-place,
+		// we must clone and add them.
+		newR := slog.NewRecord(r.Time, r.Level, r.Message, r.PC)
+		r.Attrs(func(a slog.Attr) bool {
+			newR.AddAttrs(a)
+			return true
+		})
+		newR.AddAttrs(
+			slog.String("trace_id", spanCtx.TraceID().String()),
+			slog.String("span_id", spanCtx.SpanID().String()),
+		)
+		r = newR
+	}
+
+	return h.Handler.Handle(ctx, r)
+}
+
+func (h traceContextHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	return traceContextHandler{Handler: h.Handler.WithAttrs(attrs)}
+}
+
+func (h traceContextHandler) WithGroup(name string) slog.Handler {
+	return traceContextHandler{Handler: h.Handler.WithGroup(name)}
+}
+
+// NewStdHandler returns a [slog.Handler] instance for the standard output.
+func NewStdHandler(level slog.Level, format string, fieldsPrefix string, enableGcpFields bool) (slog.Handler, error) {
+	// #nosec: G115
+	isTerminal := term.IsTerminal(int(os.Stdout.Fd()))
+
+	// Normalize the log format based on the output device.
+	if format == autoLoggingFormat {
+		if isTerminal {
+			format = textLoggingFormat
+		} else {
+			format = jsonLoggingFormat
+		}
+	}
+
+	opts := &slog.HandlerOptions{
+		Level: level,
+	}
+
+	opts.ReplaceAttr = func(groups []string, a slog.Attr) slog.Attr {
+		// Configure level encoding based on format and GCP settings.
+		if a.Key == slog.LevelKey {
+			l := a.Value.Any().(slog.Level)
+			switch {
+			case format == textLoggingFormat && isTerminal:
+				if enableGcpFields {
+					a.Value = slog.StringValue(gcpSeverityColorEncoder(l))
+				} else {
+					a.Value = slog.StringValue(levelToColor(l).Add(l.String()))
+				}
+			case enableGcpFields && format != textLoggingFormat:
+				a.Key = "severity"
+				a.Value = slog.StringValue(gcpSeverity(l))
+			default:
+				a.Value = slog.StringValue(strings.ToLower(l.String()))
+			}
+		}
+
+		if a.Key == slog.TimeKey {
+			if enableGcpFields && format != textLoggingFormat {
+				a.Key = "time"
+			} else {
+				a.Key = "ts"
+			}
+
+			if isTerminal {
+				a.Value = slog.StringValue(a.Value.Time().Local().Format("2006/01/02 15:04:05.000"))
+			} else if !enableGcpFields {
+				a.Value = slog.Float64Value(float64(a.Value.Time().UnixNano()) / 1e9)
+			}
+		}
+
+		if a.Key == slog.MessageKey {
+			if enableGcpFields && format != textLoggingFormat {
+				a.Key = "message"
+			} else {
+				a.Key = "msg"
+			}
+		}
+
+		return a
+	}
+
+	var handler slog.Handler
+	if format == textLoggingFormat {
+		handler = slog.NewTextHandler(os.Stderr, opts)
+	} else {
+		handler = slog.NewJSONHandler(os.Stderr, opts)
+	}
+
+	return traceContextHandler{Handler: handler}, nil
+}
+
+func gcpSeverityColorEncoder(l slog.Level) string {
+	severity := gcpSeverity(l)
+	c := levelToColor(l)
+	return c.Add(severity)
+}