collector/prometheus: add prometheus server (optional) as an annotation in HPA.

Signed-off-by: Eduard Ganiukov <eduard.ganiukov@swisscom.com>

README.md

@@ -172,6 +172,10 @@ kind: HorizontalPodAutoscaler
 metadata:
   name: myapp-hpa
   annotations:
+    # This annotation is optional.
+    # If specified, then this prometheus server is used,
+    # instead of the prometheus server specified as the CLI argument `--prometheus-server`.
+    metric-config.external.prometheus-query.prometheus/prometheus-server: http://prometheus.my-namespace.svc
     # metric-config.<metricType>.<metricName>.<collectorName>/<configKey>
     # <configKey> == query-name
     metric-config.external.prometheus-query.prometheus/processed-events-per-second: |

pkg/collector/prometheus_collector.go

@@ -18,8 +18,9 @@ import (
 )
 
 const (
-	PrometheusMetricName        = "prometheus-query"
-	prometheusQueryNameLabelKey = "query-name"
+	PrometheusMetricName          = "prometheus-query"
+	prometheusQueryNameLabelKey   = "query-name"
+	prometheusServerAnnotationKey = "prometheus-server"
 )
 
 type NoResultError struct {
@@ -38,7 +39,7 @@ type PrometheusCollectorPlugin struct {
 func NewPrometheusCollectorPlugin(client kubernetes.Interface, prometheusServer string) (*PrometheusCollectorPlugin, error) {
 	cfg := api.Config{
 		Address:      prometheusServer,
-		RoundTripper: &http.Transport{},
+		RoundTripper: http.DefaultTransport,
 	}
 
 	promClient, err := api.NewClient(cfg)
@@ -101,6 +102,20 @@ func NewPrometheusCollector(client kubernetes.Interface, promAPI promv1.API, hpa
 		} else {
 			return nil, fmt.Errorf("no prometheus query defined for metric")
 		}
+
+		// Use custom Prometheus URL if defined in HPA annotation.
+		if promServer, ok := config.Config[prometheusServerAnnotationKey]; ok {
+			cfg := api.Config{
+				Address:      promServer,
+				RoundTripper: http.DefaultTransport,
+			}
+
+			promClient, err := api.NewClient(cfg)
+			if err != nil {
+				return nil, err
+			}
+			c.promAPI = promv1.NewAPI(promClient)
+		}
 	}
 
 	return c, nil