Add Slack Alerts Footer

[sherifabdlnaby]

A simple Addition to Include Slack Footer in Messages

config.yml

slack_footer: "Elastic Alerts"
slack_footer_icon: "https://www.alwaysdata.com/static/img/technologies/databases/elasticsearch.png"

Results In:

[nsano-rururu]

The test is failing.
I also forgot to add an item to schema.yaml.

slack_footer: {type: string}
slack_footer_icon: {type: string}

[nsano-rururu]

After fixing the test failure, I merged it into elastalert2.