Source value in Alert is missing

[jozefbarcin]

Could you pls add this base source: generatorURL , which should be filled with graylog url

Alertmanager json example:

curl -XPOST $url -d "[{ 
	\"status\": \"firing\",
	\"labels\": {
		\"alertname\": \"$name\",
		\"service\": \"my-service\",
		\"severity\":\"warning\",
		\"instance\": \"$name.example.net\"
	},
	\"annotations\": {
		\"summary\": \"High latency is high!\"
	},
	\"generatorURL\": \"http://prometheus.int.example.net/<generating_expression>\"

Testing environment:
Graylog 3.0.2
Alermanager plugin 1.2.1
Alertmanager: 0.20.0