Debug returns a new Debug event logger.

Error returns a new Error event logger.

Fatal returns a new Fatal event logger.

Info returns a new Info event logger.

SetLogCallback sets a callback function that will be called for each log message.

Warn returns a new Warn event logger.

CallbackWriter wraps an io.Writer and calls the callback for each log entry.

Fields is a map of field names to values.

LogCallback is the type for log callback functions.