Add test and example showing how to work with istio service mesh

charts/k8s-monitoring/docs/examples/istio-service-mesh/README.md

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+<!--
+(NOTE: Do not edit README.md directly. It is a generated file!)
+(      To make changes, please modify values.yaml or description.txt and run `make examples`)
+-->
+# Istio Service Mesh example
+
+This example shows how to ensure that Alloy clustering when Istio Service Mesh is enabled and has deployed the Istio
+sidecar to the Alloy pods. This change is necessary because the Alloy cluster's headless Service port name
+[will not work](https://istio.io/latest/docs/ops/common-problems/network-issues/#503-error-while-accessing-headless-services)
+if it keeps its default port name of `http`.
+
+## Values
+
+```yaml
+---
+cluster:
+  name: istio-service-mesh-example
+
+destinations:
+  - name: localPrometheus
+    type: prometheus
+    url: http://prometheus-server.prometheus.svc:9090/api/v1/write
+
+annotationAutodiscovery:
+  enabled: true
+  annotations:
+    scrape: prometheus.io/scrape
+    metricsPath: prometheus.io/path
+    metricsPortNumber: prometheus.io/port
+
+clusterMetrics:
+  enabled: true
+
+alloy-metrics:
+  enabled: true
+  alloy:
+    clustering:
+      portName: tcp
+  controller:
+    replicas: 2
+```