SlogLogger loses error context

[oPOCCOMAXAo]

Describe the bug
In function slogErr errors are converted into string and lose metadata, like stacktrace or extra fields of error structs.

//fxevent/slog.go:270
func slogErr(err error) slog.Attr {
 	return slog.String("error", err.Error())
}

To Reproduce
Example:

package example

import (
	"context"
	"fmt"
	"log/slog"
	"testing"

	"github.com/pkg/errors"
	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
	"go.uber.org/fx"
	"go.uber.org/fx/fxevent"
)

type TestService struct{}

func NewTestService(
	lc fx.Lifecycle,
) *TestService {
	lc.Append(fx.Hook{
		OnStart: func(context.Context) error {
			return errors.WithStack(assert.AnError)
		},
	})

	return &TestService{}
}

var _ slog.Handler = (*CustomLogger)(nil)

type CustomLogger struct {
	SeenError bool
	Text      string
}

func (c *CustomLogger) Enabled(context.Context, slog.Level) bool { return true }

func (c *CustomLogger) Handle(_ context.Context, rec slog.Record) error {
	rec.Attrs(func(attr slog.Attr) bool {
		if err, ok := attr.Value.Any().(error); ok && err != nil {
			c.SeenError = true
		}

		if attr.Key == "error" {
			c.Text = attr.Value.String()
		}

		return true
	})

	return nil
}

func (c *CustomLogger) WithAttrs([]slog.Attr) slog.Handler { return c }

func (c *CustomLogger) WithGroup(string) slog.Handler { return c }

func TestExample(t *testing.T) {
	slogHandler := CustomLogger{}

	app := fx.New(
		fx.WithLogger(func() fxevent.Logger {
			return &fxevent.SlogLogger{
				Logger: slog.New(&slogHandler),
			}
		}),
		fx.Provide(NewTestService),
		fx.Invoke(func(_ *TestService, sd fx.Shutdowner) {
			sd.Shutdown()
		}),
	)

	err := app.Start(context.Background())
	require.Error(t, err, "test error")

	<-app.Done()

	assert.False(t, slogHandler.SeenError)
	assert.Equal(t, "assert.AnError general error for testing", slogHandler.Text)

	// here partial checks as path is not stable from machine to machine
	fullErrorText := fmt.Sprintf("%+v", err)
	t.Log(fullErrorText)
	assert.Contains(t, fullErrorText, "assert.AnError")
	assert.Contains(t, fullErrorText, "example_test.go:23")
	assert.Contains(t, fullErrorText, "go.uber.org/fx@v1.23.0/internal/lifecycle/lifecycle.go:265")
}

Here slog handler doesn't have access to original error, but receives untraceable string.

Original error has extra data - stacktrace, which could be retrieved in slog handler like this:

	// here partial checks as path is not stable from machine to machine
	fullErrorText := fmt.Sprintf("%+v", err)
	t.Log(fullErrorText)
	assert.Contains(t, fullErrorText, "assert.AnError")
	assert.Contains(t, fullErrorText, "example_test.go:23")
	assert.Contains(t, fullErrorText, "go.uber.org/fx@v1.23.0/internal/lifecycle/lifecycle.go:265")

Also repo with code: https://github.com/oPOCCOMAXAo/fxexample/blob/master/example_test.go

Expected behavior

Custom slog Handler must access to original error value and process it as required by business logic. Like in this example:

func (c *CustomLogger) Handle(_ context.Context, rec slog.Record) error {
	rec.Attrs(func(attr slog.Attr) bool {
		if err, ok := attr.Value.Any().(error); ok && err != nil {
			c.SeenError = true
		}

		if attr.Key == "error" {
			c.Text = attr.Value.String()
		}

		return true
	})

	return nil
}

Additional context

This will fix the issue:

//fxevent/slog.go:270
func slogErr(err error) slog.Attr {
 	return slog.Any("error", err)
}