Json formatting of the log message

[cyberroadie]

Added a configurable feature for formatting the log message as JSON instead of a string. This can be enabled by adding 'json_msg' to the configuration.

This is useful if you want to add more key-values to the error logging for future search queries in mongo.

[repeatedly]

You can use record_transformer filter with enable_ruby true for this purpose. Such record modification should be done by filter, not output plugin.