Allow Beats stack monitoring w/o elasticseachRef

pkg/controller/beat/common/stackmon/metricbeat.tpl.yml

@@ -8,6 +8,10 @@ metricbeat.modules:
   xpack.enabled: true
 http.enabled: false
 monitoring.enabled: false
-monitoring.cluster_uuid: "{{ .ClusterUUID }}"
+# if no associated Elasticsearch cluster UUID known or determinable,
+# monitored application will be treated as stand-alone without cluster_uuid.
+{{ with .ClusterUUID -}}
+monitoring.cluster_uuid: "{{ . }}"
+{{- end}}
 
 # Elasticsearch output configuration is generated and added here

pkg/controller/beat/common/stackmon/stackmon.go

@@ -128,6 +128,10 @@ type clusterUUIDResponse struct {
 func associatedESUUID(ctx context.Context, client k8s.Client, beat *v1beta1.Beat) (string, error) {
 	esAssociation := beat.EsAssociation()
 	esRef := esAssociation.AssociationRef()
+	if !esRef.IsDefined() {
+		// no association or indirect association e.g. via output configuration
+		return "", nil
+	}
 	if esRef.IsExternal() {
 		remoteES, err := association.GetUnmanagedAssociationConnectionInfoFromSecret(client, esAssociation)
 		if err != nil {

pkg/controller/beat/common/stackmon/stackmon_test.go

@@ -111,6 +111,30 @@ output:
         ssl:
             verification_mode: certificate
         username: es-user
+`
+	standaloneBeatYML := `http:
+    enabled: false
+metricbeat:
+    modules:
+        - hosts:
+            - http+unix:///var/shared/metricbeat-test-beat.sock
+          metricsets:
+            - stats
+            - state
+          module: beat
+          period: 10s
+          xpack:
+            enabled: true
+monitoring:
+    enabled: false
+output:
+    elasticsearch:
+        hosts:
+            - es-metrics-monitoring-url
+        password: es-password
+        ssl:
+            verification_mode: certificate
+        username: es-user
 `
 	beatSidecarFixture := func(beatYml string) stackmon.BeatSidecar {
 		return stackmon.BeatSidecar{
@@ -251,6 +275,23 @@ output:
 			want:    beatSidecarFixture(fmt.Sprintf(beatYml, "abcd1234", "es-metrics-monitoring-url")),
 			wantErr: false,
 		},
+		{
+			name: "beat with stack monitoring enabled and no elasticsearchRef returns configured sidecar",
+			args: args{
+				client: k8s.NewFakeClient(&beatFixture, &esFixture, &monitoringEsFixture, &corev1.Secret{
+					ObjectMeta: metav1.ObjectMeta{Name: "es-secret-name", Namespace: "test"},
+					Data:       map[string][]byte{"es-user": []byte("es-password")},
+				}),
+				beat: func() *v1beta1.Beat {
+					beat := beatFixture.DeepCopy()
+					beat.Spec.ElasticsearchRef = commonv1.ObjectSelector{}
+					return beat
+				},
+				version: "8.2.3",
+			},
+			want:    beatSidecarFixture(standaloneBeatYML),
+			wantErr: false,
+		},
 		{
 			name: "Beat with stack monitoring enabled and remote elasticsearchRef",
 			args: args{