Add example with condition using where entityname

Please add an alert example where the condition includes an entity name. I couldn't find an example anywhere and this seems to be quite a useful condition to use.

For example
az monitor metrics alert create -n "Event Hub Output Flatline - {EventHub}" --target-resource-type Microsoft.EventHub/namespaces --resource-group {ResourceGroup} \
--scopes {EventHubNamespaceID} \
--condition "avg OutgoingMessages < 100 where EntityName includes {EventHub}" \

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* ID: d97968df-72c9-e50c-d5b1-54562c1bdef7
* Version Independent ID: 0cde4f15-6464-3106-b787-3ee37c7de0ee
* Content: [az monitor metrics alert](https://docs.microsoft.com/en-us/cli/azure/monitor/metrics/alert?view=azure-cli-latest#az_monitor_metrics_alert_create)
* Content Source: [latest/docs-ref-autogen/monitor/metrics/alert.yml](https://github.com/MicrosoftDocs/azure-docs-cli/blob/master/latest/docs-ref-autogen/monitor/metrics/alert.yml)
* Service: **monitoring-and-diagnostics**
* GitHub Login: @rloutlaw
* Microsoft Alias: **routlaw**