Add TriggerLevelWriter

[mitar]

Fixes #583.

[mitar]

In my own code, I then made:

func WithContext(ctx context.Context, writer io.Writer, level, conditionalLevel, triggerLevel zerolog.Level) (context.Context, func(), func()) {
	w := zerolog.NewTriggerLevelWriter(
		writer,
		conditionalLevel,
		triggerLevel,
	)
	ctxLogger := zerolog.New(w).Level(level).With().Timestamp().Logger()
	closeCtx := func() {
		_ = w.Close()
	}
	trigger := func() {
		_ = w.Trigger()
	}
	return ctxLogger.WithContext(ctx), closeCtx, trigger
}

One has then to call closeCtx after the context is not used anymore.

And I then have:

func NewHandler(writer io.Writer, level, conditionalLevel, triggerLevel zerolog.Level)) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
			ctx, closeCtx, trigger := WithContext(req.Context(), writer, level, conditionalLevel, triggerLevel)
			defer closeCtx()
			panicking := true
			defer func() {
				if panicking {
					trigger()
				}
			}()
			req = req.WithContext(ctx)
			next.ServeHTTP(w, req)
			panicking = false
		})
	}
}

You can find them in my zerolog wrapper package.

[mitar]

@rs: I addressed your review comments.