fix discord & msteams webhook url configuration (prometheus#3728)

Signed-off-by: Luca Kröger <l.kroeger01@gmail.com>
Signed-off-by: Gokhan Sari <gokhan@sari.me>

config/config.go

@@ -516,8 +516,8 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 			if discord.HTTPConfig == nil {
 				discord.HTTPConfig = c.Global.HTTPConfig
 			}
-			if discord.WebhookURL == nil {
-				return fmt.Errorf("no discord webhook URL provided")
+			if discord.WebhookURL == nil && len(discord.WebhookURLFile) == 0 {
+				return fmt.Errorf("no discord webhook URL or URLFile provided")
 			}
 		}
 		for _, webex := range rcv.WebexConfigs {
@@ -536,8 +536,8 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 			if msteams.HTTPConfig == nil {
 				msteams.HTTPConfig = c.Global.HTTPConfig
 			}
-			if msteams.WebhookURL == nil {
-				return fmt.Errorf("no msteams webhook URL provided")
+			if msteams.WebhookURL == nil && len(msteams.WebhookURLFile) == 0 {
+				return fmt.Errorf("no msteams webhook URL or URLFile provided")
 			}
 		}