Implement slog.LogValuer

[samber]

The new slog library provides a slog.LogValuer interface for formatting attributes.

Documentation here: https://pkg.go.dev/golang.org/x/exp/slog#hdr-Customizing_a_type_s_logging_behavior

I think cockroachdb/errors should implement it.

type customError struct {
    ...
}

func (e customError) Error() string {
    ...
}

// implements slog.LogValuer
func (e customError) LogValue() slog.Value {
	return slog.GroupValue(
	        slog.Int("code", e.code),
	        slog.String("message", e.msg),
	        slog.String("stacktrace", e.stacktrace),
        )
}

slog will be released in go 1.21. But is already available and golang.org/x/exp/slog

[nozo-moto]

@knz @dhartunian
Hi maintainers! I've recently implemented the slog.LogValuer feature in our project. I'd appreciate your input on the code quality, documentation, and any potential improvements. Please feel free to comment whenever you have time. Thank you!
#129