feat(slack) add rate limiting

internal/service/slack.go

@@ -5,23 +5,38 @@ import (
 
 	"github.com/PDOK/uptime-operator/internal/model"
 	"github.com/slack-go/slack"
+	"golang.org/x/time/rate"
 	"sigs.k8s.io/controller-runtime/pkg/log"
 )
 
+const (
+	nrOfMessagesPerSec   = 1
+	nrOfMessagesPerBurst = 10
+)
+
 type Slack struct {
 	webhookURL string
 	channelID  string
+
+	rateLimit *rate.Limiter
 }
 
 func NewSlack(webhookURL, channelID string) *Slack {
 	return &Slack{
 		webhookURL: webhookURL,
 		channelID:  channelID,
+
+		// see https://api.slack.com/apis/rate-limits
+		rateLimit: rate.NewLimiter(nrOfMessagesPerSec, nrOfMessagesPerBurst),
 	}
 }
 
 func (s *Slack) Send(ctx context.Context, message string) {
-	err := slack.PostWebhook(s.webhookURL, &slack.WebhookMessage{
+	err := s.rateLimit.Wait(ctx)
+	if err != nil {
+		log.FromContext(ctx).Error(err, "failed waiting for slack rate limit")
+	}
+	err = slack.PostWebhook(s.webhookURL, &slack.WebhookMessage{
 		Channel:   s.channelID,
 		Text:      message,
 		Username:  model.OperatorName,