Proposal

This is a bit of a bug/missing feature. When Azure AD auth support for remote write was added (#11733), I think they missed that what prometheus config calls the "client_id" is optional (https://github.com/prometheus/prometheus/blob/5b35381e21931ebc9baa6149347a4140b158f96b/storage/remote/azuread/azuread.go#L114-116). If you don't pass an "ManagedIdentityCredentialOptions.ID" into NewManagedIdentityCredential , it creates a credential for the System Managed Identity (https://pkg.go.dev/github.com/Azure/azure-sdk-for-go/sdk/azidentity#ManagedIdentityCredential). Currently, since the client_id is required, you can only use Azure AD User Assigned Managed Identities with prometheus remote write, and not Azure AD System Managed Identities, like I want.

This seems like a pretty straightforwards change (just make the prometheus config optional), so I'd like to try my hand at a PR.