Azure Container Service: Diagnostics Clarification


:wave: Me again

I've been working with Azure Container Service recently - [adding support into Terraform](https://github.com/hashicorp/terraform/pull/10820) and I've opened a few issues regarding issues I've noticed.

This issue is related to the Diagnostics Profile on a Container Service instance - where it's currently not possible to set the value for the storage uri field. This is both a server-side (API) and client-side (SDK) limitation.

Given the storage uri field is read-only - I initially assumed that when enabling Diagnostics a Storage Account may be created - thus hence the Read Only field. However when creating an instance and enabling diagnostics - the storage_uri field in the Diagnostics Profile is null.

Next I assumed this might be a bug in the SDK - so I changed my local SDK to ignore the validation and make the request anyway - where it's rejected by the server.

Clearly this is how the feature's desired at this point - so my questions are:
- Why expose the field as read-only if it's only ever going to be null?
- If it's expected to be set by the user - shouldn't it be possible to set this field both in the API and the SDK?

Feel free to redirect me if I'm filing bugs in the wrong place - but this seemed like the most sensible place to file it :)

Thanks!