Use fmt.Sprintf instead of string concatenate

[mxyng]

While fmt.Sprintf() may be less performant than string concatenate, it's a nicer reading experience. At our current scale, the performance hit shouldn't be relevant. For logging purposes though, we should be using the zap.Logger.Sugar().Infof() instead since should evaluate the log statement unless it's configured to log; fmt.Sprintf() will evaluate the string regardless of the log level which means it could be doing unnecessary work.