[Question] What is the purpose of Event Grid `SystemTopicProperties.metricResourceId`?

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/eventgrid/resource-manager/Microsoft.EventGrid/stable/2025-02-15/EventGrid.json#L15488

### API Spec version

2025-02-15

### Question/Query

What is the significance, purpose, or use of the metricResourceId property? Its description in the spec is:

> Metric resource id for the system topic.

In practice, it seems to always be a GUID. However, I cannot find any practical use for this identifier in Azure or anywhere else. What was the intention or application of this identifier? I supposed Azure Monitor could accept the value, but I was unable to form a valid request using it, requiring instead the Azure RM identifier ("/subscriptions/0000...").

Here is an entire Event Grid System Topic JSON i created, showing the property in context:

```json
{
"properties": {
"provisioningState": "Succeeded",
"source": "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/my-group/providers/Microsoft.Storage/storageAccounts/my-account",
"topicType": "Microsoft.Storage.StorageAccounts",
"metricResourceId": "84003d31-46be-4***-a***-************"
},
"systemData": null,
"location": "westeurope",
"id": "/subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/my-group/providers/Microsoft.EventGrid/systemTopics/my-topic",
"name": "my-topic",
"type": "Microsoft.EventGrid/systemTopics"
}
```

### Environment

_No response_

Start with specification/eventgrid/resource-manager/Microsoft.EventGrid/stable/2025-02-15/EventGrid.json at line 15488 and inspect the SystemTopicProperties definition in context. Check the Event Grid and Azure Monitor documentation for how metricResourceId is intended to be used. Done means providing an authoritative explanation or updating the property description with its purpose and expected identifier format.

Written by the indexing model from the issue text.