Add observability samples (self managed) for Istio addon

examples/istio-based-service-mesh/observability/README.md

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+# Telemetry samples
+
+The addon already have native integration with Azure managed Prometheus and managed Grafana etc. We strongly suggest Azure customers use these managed solutions.
+
+Based on artifacts from open source community, this directory contains sample deployments of observability in a self-managed manner.
+
+## Prometheus + Grafana
+
+### Install
+
+```shell
+kubectl apply prometheus.yaml
+kubectl apply grafana.yaml
+```
+
+### Test Grafana
+
+```shell
+kubectl -n aks-istio-system port-forward service/grafana 3000:3000
+```
+
+Open http://localhost:3000 in your browser to monitor the health status of your mesh.