slog_with_tracectx.go

package main

import (
	"context"
	"log/slog"
	"os"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/exporters/stdout/stdouttrace"
	"go.opentelemetry.io/otel/sdk/resource"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	semconv "go.opentelemetry.io/otel/semconv/v1.7.0"
	"go.opentelemetry.io/otel/trace"
)

var (
	tracer = otel.Tracer("main")
)

type CustomHandler struct {
	slog.Handler
}

func (h *CustomHandler) Handle(ctx context.Context, r slog.Record) error {
	span := trace.SpanFromContext(ctx)
	if span.SpanContext().IsValid() {
		r.AddAttrs(slog.String("traceID", span.SpanContext().TraceID().String()))
		r.AddAttrs(slog.String("spanID", span.SpanContext().SpanID().String()))
	}

	return h.Handler.Handle(ctx, r)
}

func main() {
	handler := &CustomHandler{Handler: slog.NewJSONHandler(os.Stdout, nil)}
	logger := slog.New(handler)
	slog.SetDefault(logger)

	cleanup, err := setupTraceProvider("example-service", "1.0.0")
	if err != nil {
		panic(err)
	}
	defer cleanup()

	ctx, span := tracer.Start(context.Background(), "main")
	defer span.End()

	someFunc(ctx)
}

func someFunc(ctx context.Context) {
	ctx, span := tracer.Start(ctx, "someFunc")
	defer span.End()

	slog.InfoContext(ctx, "someFunc called")
}

func setupTraceProvider(serviceName string, serviceVersion string) (func(), error) {

	// write to file
	file, err := os.OpenFile("trace.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)

	exporter, err := stdouttrace.New(
		stdouttrace.WithPrettyPrint(),
		stdouttrace.WithWriter(file),
	)
	if err != nil {
		return nil, err
	}

	resource := resource.NewWithAttributes(
		semconv.SchemaURL,
		semconv.ServiceNameKey.String(serviceName),
		semconv.ServiceVersionKey.String(serviceVersion),
	)

	tracerProvider := sdktrace.NewTracerProvider(
		sdktrace.WithBatcher(exporter),
		sdktrace.WithSampler(sdktrace.AlwaysSample()),
		sdktrace.WithResource(resource),
	)
	otel.SetTracerProvider(tracerProvider)

	cleanup := func() {
		ctx, cancel := context.WithCancel(context.Background())
		defer cancel()
		tracerProvider.Shutdown(ctx)
	}
	return cleanup, nil
}