Add User-Agent for webhook requests (#893)
Add User-Agent for webhook requests (#893)


prometheus/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,tinytub/alertmanager,Aloomaio/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,joehandzik/alertmanager,prometheus/alertmanager,prometheus/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,tinytub/alertmanager,joehandzik/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,tinytub/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,tyrken/alertmanager,Kellel/alertmanager,Aloomaio/alertmanager