Add istio control plane dashboard and alert rule (#478)

* Add istio control plane dashboard

The source of this dashboard is [1].

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[1]: https://grafana.com/grafana/dashboards/7645-istio-control-plane-dashboard/

* Add aler rule from source [1] based on metrics [2]

Found some intteresting alert rules in [1], however based on istio
doc [2], only the last one will work, so I added it here.

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[1]: https://samber.github.io/awesome-prometheus-alerts/rules#istio
[2]: https://istio.io/latest/docs/reference/commands/pilot-discovery/#metrics

charms/istio-gateway/src/prometheus_alert_rules/basic.rules

@@ -7,7 +7,7 @@ groups:
           summary: 'ingress gateway traffic missing'
           description: '[Critical]: ingress gateway traffic missing, likely other monitors are misleading, check client logs'
         expr: >
-          absent(reporter="source", source_workload=~"istio-(ingress|egress)gateway-workload"})==1
+          absent(istio_requests_total{reporter="source", source_workload=~"istio-(ingress|egress)gateway-workload"})==1
         for: 5m
       - alert: IstioMetricsMissing
         annotations: