Don't generate any metrics bits if prometheus is disabled

If observability is completely disabled, we still add to the generated
k8s manifest file the metrics port on which we export metrics, and we
also create a label neede by prometheus to scrape the pods.

This PR disables generating such info unless prometheus is enabled.

internal/impl/kube.go

@@ -189,14 +189,17 @@ func (r *replicaSet) buildDeployment(cfg *KubeConfig) (*appsv1.Deployment, error
 	podLabels := map[string]string{
 		"appName": r.app.Name,
 		"depName": name,
-		"metrics": r.app.Name, // Needed by Prometheus to scrape the metrics.
 	}
+	if cfg.Observability[metricsConfigKey] != disabled {
+		podLabels["metrics"] = r.app.Name // Needed by Prometheus to scrape the metrics.
+	}
+
 	dnsPolicy := corev1.DNSClusterFirst
 	if cfg.UseHostNetwork {
 		dnsPolicy = corev1.DNSClusterFirstWithHostNet
 	}
 
-	container, err := r.buildContainer()
+	container, err := r.buildContainer(cfg)
 	if err != nil {
 		return nil, err
 	}
@@ -320,7 +323,7 @@ func (r *replicaSet) buildAutoscaler() (*autoscalingv2.HorizontalPodAutoscaler,
 }
 
 // buildContainer builds a container specification for a replica set.
-func (r *replicaSet) buildContainer() (corev1.Container, error) {
+func (r *replicaSet) buildContainer(cfg *KubeConfig) (corev1.Container, error) {
 	// Set the binary path in the deployment w.r.t. to the binary path in the
 	// docker image.
 	r.app.Binary = fmt.Sprintf("/weaver/%s", filepath.Base(r.app.Binary))
@@ -339,9 +342,7 @@ func (r *replicaSet) buildContainer() (corev1.Container, error) {
 
 	// Always expose the metrics port from the container, so it can be
 	// discoverable for scraping by Prometheus.
-	ports := []corev1.ContainerPort{
-		{Name: "prometheus", ContainerPort: defaultMetricsPort},
-	}
+	var ports []corev1.ContainerPort
 	// Expose all of the listener ports.
 	for _, ls := range r.components {
 		for _, l := range ls.Listeners {
@@ -352,6 +353,17 @@ func (r *replicaSet) buildContainer() (corev1.Container, error) {
 		}
 	}
 
+	if cfg.Observability[metricsConfigKey] != disabled {
+		// Expose the metrics port from the container, so it can be discoverable for
+		// scraping by Prometheus.
+		// TODO(rgrandl): We may want to have a default metrics port that can be scraped
+		// by any metrics collection system. For now, disable the port if Prometheus
+		// will not collect the metrics.
+		ports = append(ports, corev1.ContainerPort{
+			Name: "prometheus", ContainerPort: defaultMetricsPort,
+		})
+	}
+
 	return corev1.Container{
 		Name:            appContainerName,
 		Image:           r.image,