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

[wyattfry]

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:

{
    "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