Original slack output plugin can just send whole data and does not support formatting.
[OUTPUT]
Name prettyslack
Match kernel.slack
Webhook https://hooks.slack.com/services/TXXXXXXXXXX/BXXXXXXXXX/XXXXXXXXXXXXXX
Channel myChannel
Message_Field msg
Title logs from ${NODE_NAME}
Max_Attachments 7
Webhook : slack incomming webhook url
Channel : slack channel name
Title : slack message title
Message_Field : key for first line of slack message
Max_Attachments : number of max attachments of 1 slack message. ( Defaut : 5, Max : 20)
Color value of attachment from data with key 'color' (Default : #A9AAAA)