make metricsServerOptions configurable and add documentation

Author: bruelea

README.md

@@ -87,6 +87,16 @@ Users can just run `kubectl apply -f <URL for YAML BUNDLE>` to install the proje
 kubectl apply -f https://raw.githubusercontent.com/<org>/netbox-operator/<tag or branch>/dist/install.yaml
 ```
 
+# Monitoring
+
+When the operator is deployed with the default kustomization (located at config/default/) the metrics endpoint is already exposed and provides the [default kubebuilder metrics].
+
+[default kubebuilder metrics]: https://book.kubebuilder.io/reference/metrics-reference.
+
+For the monitoring of the state of the CRs reconciled by the operator [kube state metrics] can be used, check the kube-state-metrics documentation for instructions on configuring it to collect metrics from custom resources.
+
+[kube state metrics]: https://github.com/kubernetes/kube-state-metrics
+
 # Contributing
 
 We cordially invite collaboration from the community to enhance the quality and functionality of this project. Whether you are addressing bugs, introducing new features, refining documentation, or assisting with items on our to-do list, your contributions are highly valued and greatly appreciated. Please take a look at [Contribution guide] for more details.

cmd/main.go

@@ -64,12 +64,14 @@ func main() {
 	var probeAddr string
 	var secureMetrics bool
 	var enableHTTP2 bool
-	flag.StringVar(&metricsAddr, "metrics-bind-address", ":8080", "The address the metric endpoint binds to.")
+	var tlsOpts []func(*tls.Config)
+	flag.StringVar(&metricsAddr, "metrics-bind-address", "0", "The address the metrics endpoint binds to. "+
+		"Use :8443 for HTTPS or :8080 for HTTP, or leave as 0 to disable the metrics service.")
 	flag.StringVar(&probeAddr, "health-probe-bind-address", ":8081", "The address the probe endpoint binds to.")
 	flag.BoolVar(&enableLeaderElection, "leader-elect", false,
 		"Enable leader election for controller manager. "+
 			"Enabling this will ensure there is only one active controller manager.")
-	flag.BoolVar(&secureMetrics, "metrics-secure", false,
+	flag.BoolVar(&secureMetrics, "metrics-secure", true,
 		"If set the metrics endpoint is served securely")
 	flag.BoolVar(&enableHTTP2, "enable-http2", false,
 		"If set, HTTP/2 will be enabled for the metrics and webhook servers")
@@ -92,23 +94,40 @@ func main() {
 		c.NextProtos = []string{"http/1.1"}
 	}
 
-	tlsOpts := make([]func(*tls.Config), 0, 1)
 	if !enableHTTP2 {
 		tlsOpts = append(tlsOpts, disableHTTP2)
 	}
 
 	webhookServer := webhook.NewServer(webhook.Options{
 		TLSOpts: tlsOpts,
 	})
+	// Metrics endpoint is enabled in 'config/default/kustomization.yaml'. The Metrics options configure the server.
+	// More info:
+	// - https://pkg.go.dev/sigs.k8s.io/controller-runtime@v0.19.0/pkg/metrics/server
+	// - https://book.kubebuilder.io/reference/metrics.html
+	metricsServerOptions := metricsserver.Options{
+		BindAddress:   metricsAddr,
+		SecureServing: secureMetrics,
+		// TODO(user): TLSOpts is used to allow configuring the TLS config used for the server. If certificates are
+		// not provided, self-signed certificates will be generated by default. This option is not recommended for
+		// production environments as self-signed certificates do not offer the same level of trust and security
+		// as certificates issued by a trusted Certificate Authority (CA). The primary risk is potentially allowing
+		// unauthorized access to sensitive metrics data. Consider replacing with CertDir, CertName, and KeyName
+		// to provide certificates, ensuring the server communicates using trusted and secure certificates.
+		TLSOpts: tlsOpts,
+	}
+
+	if secureMetrics {
+		// FilterProvider is used to protect the metrics endpoint with authn/authz.
+		// These configurations ensure that only authorized users and service accounts
+		// can access the metrics endpoint. The RBAC are configured in 'config/rbac/kustomization.yaml'. More info:
+		// https://pkg.go.dev/sigs.k8s.io/controller-runtime@v0.19.0/pkg/metrics/filters#WithAuthenticationAndAuthorization
+		metricsServerOptions.FilterProvider = filters.WithAuthenticationAndAuthorization
+	}
 
 	mgr, err := ctrl.NewManager(ctrl.GetConfigOrDie(), ctrl.Options{
-		Scheme: scheme,
-		Metrics: metricsserver.Options{
-			BindAddress:    metricsAddr,
-			SecureServing:  secureMetrics,
-			TLSOpts:        tlsOpts,
-			FilterProvider: filters.WithAuthenticationAndAuthorization,
-		},
+		Scheme:                 scheme,
+		Metrics:                metricsServerOptions,
 		WebhookServer:          webhookServer,
 		HealthProbeBindAddress: probeAddr,
 		LeaderElection:         enableLeaderElection,