[otelzap] Extra fields config not used in sugared logger

[skubakdj]

Hey! Thank you for this awesome library!

I would like to include extra fields on my otelzap logger with every message. To do that I'm looking at the otelzap.WithExtraFields() config and also maybe using .With() in the sugared logger.

I'm finding the otelzap.WithExtraFields() config to be respected in the plain logger, but it doesn't look to be used in the sugared logger. Also, using With() in the sugared logger doesn't seem to work when chained with Ctx().

Here's some code to illustrate the above:

package main

import (
	"context"
	"log"

	"github.com/uptrace/opentelemetry-go-extra/otelzap"
	"go.uber.org/zap"
	"go.uber.org/zap/zapcore"
)

func main() {

	// Create new Zap logger
	var level zapcore.Level
	err := level.UnmarshalText([]byte("INFO"))
	if err != nil {
		log.Fatal(err)
	}
	config := zap.NewProductionConfig()
	config.Level = zap.NewAtomicLevelAt(level)
	zapLogger, err := config.Build(zap.AddCallerSkip(0))
	if err != nil {
		log.Fatal(err)
	}

	// Create OTEL Zap logger
	otelZapLogger := otelzap.New(
		zapLogger,
		otelzap.WithMinLevel(zapLogger.Level()),
		otelzap.WithExtraFields(zap.String("extra_key", "extra_value")),
	)

	ctx := context.Background()

	// Example scenarios
	otelZapLogger.Ctx(ctx).Info("plain + ctx, DOES have extra fields")

	otelZapLogger.Sugar().Ctx(ctx).Infof("sugared + ctx, DOES NOT have extra fields")

	otelZapLogger.Sugar().With("with_key", "with_value").Infof("sugared + with, DOES have 'with' fields, DOES NOT have extra fields")

	otelZapLogger.Sugar().With("with_key", "with_value").Ctx(ctx).Infof("sugared + with + ctx, DOES NOT have 'with' fields, DOES NOT have extra fields")

	otelZapLogger.Sugar().With("with_key", "with_value").Ctx(ctx).Desugar().Info("desugared + with + ctx, DOES NOT have 'with' fields, DOES have extra fields")
}

This produces the following output:

{"level":"info","ts":1721846112.587265,"caller":"test_logger/main.go:37","msg":"plain + ctx, DOES have extra fields","extra_key":"extra_value"}
{"level":"info","ts":1721846112.587362,"caller":"test_logger/main.go:39","msg":"sugared + ctx, DOES NOT have extra fields"}
{"level":"info","ts":1721846112.587384,"caller":"test_logger/main.go:41","msg":"sugared + with, DOES have 'with' fields, DOES NOT have extra fields","with_key":"with_value"}
{"level":"info","ts":1721846112.587411,"caller":"test_logger/main.go:43","msg":"sugared + with + ctx, DOES NOT have 'with' fields, DOES NOT have extra fields"}
{"level":"info","ts":1721846112.587422,"caller":"test_logger/main.go:45","msg":"desugared + with + ctx, DOES NOT have 'with' fields, DOES have extra fields","extra_key":"extra_value"}

Is this a bug, a limitation, or is there some other way to use the sugared logger?