Add auto detection for Prometheus server service

cmd/icinga-kubernetes/main.go

@@ -126,6 +126,13 @@ func main() {
 			klog.Error(errors.Wrap(err, "cannot retrieve prometheus config"))
 
 		}
+
+		if cfg.Prometheus.Url == "" {
+			err = metrics.AutoDetectPrometheus(ctx, clientset, &cfg.Prometheus)
+			if err != nil {
+				klog.Error(errors.Wrap(err, "cannot auto-detect prometheus"))
+			}
+		}
 	}
 
 	if cfg.Prometheus.Url != "" {

pkg/metrics/config.go

@@ -2,9 +2,12 @@ package metrics
 
 import (
 	"context"
+	"fmt"
 	"github.com/icinga/icinga-go-library/database"
 	schemav1 "github.com/icinga/icinga-kubernetes/pkg/schema/v1"
 	"github.com/pkg/errors"
+	kmetav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/client-go/kubernetes"
 )
 
 // PrometheusConfig defines Prometheus configuration.
@@ -52,3 +55,28 @@ func RetrievePrometheusConfig(ctx context.Context, db *database.DB, config *Prom
 
 	return nil
 }
+
+func AutoDetectPrometheus(ctx context.Context, clientset *kubernetes.Clientset, config *PrometheusConfig) error {
+	services, err := clientset.CoreV1().Services("monitoring").List(ctx, kmetav1.ListOptions{
+		LabelSelector: "app.kubernetes.io/component=server",
+	})
+	if err != nil {
+		return errors.Wrap(err, "cannot list Prometheus services")
+	}
+
+	if len(services.Items) == 0 {
+		return errors.New("no Prometheus service found")
+	}
+
+	if len(services.Items) > 1 {
+		return errors.New("multiple Prometheus services found")
+	}
+
+	config.Url = fmt.Sprintf(
+		"http://%s:%d",
+		services.Items[0].Spec.ClusterIP,
+		services.Items[0].Spec.Ports[0].Port,
+	)
+
+	return nil
+}