I think Uber chose a better approach for their Go logging library called Zap [1]
logger.Info("failed to fetch URL",
// Structured context as strongly typed Field values.
zap.String("url", url),
zap.Int("attempt", 3),
zap.Duration("backoff", time.Second),
)
They also have zap.Error(err) which generates the "error" key as a convention.