v4: Support for context-based structured logging

[diamondburned]

Arikawa should (by now) all have context APIs that would allow the user to inject values into contexts. We could use this to inject our own structured logger in that would allow the user to easily debug their bot.

The logging should not deviate too far from stdlib log so we can maintain compatibility.

Proposed API:

package logger // import "github.com/diamondburned/arikawa/v4/utils/logger"

// Logger is a logger that prints logs. It must be safe for concurrent use.
type Logger interface {
	// WithValue returns a new Logger with the given key-value pair. No visual
	// information is required to be shown.
    WithValue(name string, value any) Logger
	// WithPrefix returns a new Logger with the given prefix. The prefix must be
	// shown before the log message.
	WithPrefix(prefix string) Logger

	Print(v ...any)
	Printf(format string, v ...any)
	Println(v ...any)
}

type stdLogger struct {
	*log.Logger
}

// NewStandardLogger returns a new Logger that uses the standard library's log
// package.
func NewStandardLogger(logger *log.Logger) Logger {
	return stdLogger{logger}
}

// DefaultLogger is the default logger used by the package. It is a standard
// library logger that prints to os.Stderr.
var DefaultLogger = NewStandardLogger(log.Default())

func (p stdLogger) WithValue(name string, value any) Logger {
	return p
}

func (p stdLogger) WithPrefix(prefix string) Logger {
	logger := log.New(p.Writer(), p.Prefix() + ": " + prefix, p.Flags())
	return stdLogger{logger}
}

// FromContext returns a Logger from the given context. If no Logger is found,
// the DefaultLogger is returned.
func FromContext(ctx context.Context) Logger {
	if logger, ok := ctx.Value(loggerKey).(Logger); ok {
		return logger
	}
	return DefaultLogger
}

Usage:

// setup
ctx = logger.WithPrefix(ctx, "gateway")
ctx = logger.WithValue(ctx, "shard", shard)
ctx = logger.WithValue(ctx, "event", event)
// use
log := logger.FromContext(ctx)
log.Println("cannot handle interaction:", err)

[diamondburned]

We could use x/exp/slog.