feat(alerting): Add message-content parameter for Discord pings (#1335)

* feat(discord-alerts): add option for prefix-messages outside of embeds

* chore(docs): add discord prefix-message to README

* chore(discord-alerts): rename prefix-message to message-content

---------

Co-authored-by: TwiN <twin@linux.com>

alerting/provider/discord/discord.go

@@ -20,8 +20,9 @@ var (
 )
 
 type Config struct {
-	WebhookURL string `yaml:"webhook-url"`
-	Title      string `yaml:"title,omitempty"` // Title of the message that will be sent
+	WebhookURL     string `yaml:"webhook-url"`
+	Title          string `yaml:"title,omitempty"`           // Title of the message that will be sent
+	MessageContent string `yaml:"message-content,omitempty"` // Message content for pinging users or groups (e.g. "<@123456789>" or "<@&987654321>")
 }
 
 func (cfg *Config) Validate() error {
@@ -38,6 +39,9 @@ func (cfg *Config) Merge(override *Config) {
 	if len(override.Title) > 0 {
 		cfg.Title = override.Title
 	}
+	if len(override.MessageContent) > 0 {
+		cfg.MessageContent = override.MessageContent
+	}
 }
 
 // AlertProvider is the configuration necessary for sending an alert using Discord
@@ -142,7 +146,7 @@ func (provider *AlertProvider) buildRequestBody(cfg *Config, ep *endpoint.Endpoi
 		title = cfg.Title
 	}
 	body := Body{
-		Content: "",
+		Content: cfg.MessageContent,
 		Embeds: []Embed{
 			{
 				Title:       title,