Update description for metric alert API to clarify scope limitations

[shirelch1]

ARM (Control Plane) API Specification Update Pull Request

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PR review workflow diagram

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Purpose of this PR

What's the purpose of this PR? Check the specific option that applies. This is mandatory!

• New resource provider.
• New API version for an existing resource provider. (If API spec is not defined in TypeSpec, the PR should have been created in adherence to OpenAPI specs PR creation guidance).
• Update existing version for a new feature. (This is applicable only when you are revising a private preview API version.)
• Update existing version to fix OpenAPI spec quality issues in S360.
• Convert existing OpenAPI spec to TypeSpec spec (do not combine this with implementing changes for a new API version).
• Other, please clarify:
  • added a note to the CLI documentation

Due diligence checklist

To merge this PR, you must go through the following checklist and confirm you understood
and followed the instructions by checking all the boxes:

• I confirm this PR is modifying Azure Resource Manager (ARM) related specifications, and not data plane related specifications.
• I have reviewed following Resource Provider guidelines, including
  ARM resource provider contract and
  REST guidelines (estimated time: 4 hours).
  I understand this is required before I can proceed to the diagram Step 2, "ARM API changes review", for this PR.

Additional information

Viewing API changes

For convenient view of the API changes made by this PR, refer to the URLs provided in the table
in the Generated ApiView comment added to this PR. You can use ApiView to show API versions diff.

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Getting help

• First, please carefully read through this PR description, from top to bottom. Please fill out the Purpose of this PR and Due diligence checklist.
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[openapi-pipeline-app]

Next Steps to Merge

Next steps that must be taken to merge this PR:

• ❌ This PR is in purview of the ARM review (label: ARMReview). This PR must get ARMSignedOff label from an ARM reviewer.
  This PR is awaiting ARM reviewer feedback (label: WaitForARMFeedback).
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• ❌ The required check named Swagger BreakingChange has failed. To unblock this PR, follow the process at aka.ms/brch.

[openapi-pipeline-app]

Generated ApiView

Language	Package Name	ApiView Link
Go	sdk/resourcemanager/monitor/armmonitor	There is no API change compared with the previous version
Java	azure-resourcemanager-monitor-generated	https://apiview.dev/Assemblies/Review/c5eba455f3ab4ce4bf0c0a2da8091fce?revisionId=ce53f26d06624dd0b458bc923489aa74
JavaScript	@azure/arm-monitor	https://apiview.dev/Assemblies/Review/6bb257ec6c7a48e3812505db443df4ae?revisionId=fd45066d1fc24d23a730707f05983493

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

Microsoft.Insights