docs: Add new Autoscaling Azure VMSS target plugin detail. (#9186)

Co-authored-by: Luiz Aoqui <luiz@hashicorp.com>

website/pages/docs/autoscaling/plugins/target.mdx

@@ -11,6 +11,12 @@ Target Plugins determine where the resource to be autoscaled is located. All
 target plugins support the `dry-run` policy config parameter which allows a policy
 to be evaluated, but will noop any suggested changes.
 
+Below is a list of plugins you can use with the Nomad Autoscaler:
+- [Nomad Task Group][nomad_task_group_target]
+- [Dynamic Application Sizing Nomad Task][das_task_target]
+- [AWS AutoScaling Group][aws_asg_target]
+- [Azure Virtual Machine Scale Set][azure_vmss_target]
+
 ## Nomad Task Group Target
 
 The Nomad task group target indicates the scalable resource is a Nomad job
@@ -214,6 +220,118 @@ check "hashistack-allocated-cpu" {
 - `node_purge` `(bool: "false")` A boolean flag to determine whether Nomad clients should
   be [purged][nomad_node_purge] when performing scale in actions.
 
+## Azure Virtual Machine Scale Set Target
+
+The Azure VMSS target plugin allows for the scaling of the Nomad cluster clients
+via manipulating [Azure Virtual Machine Scale Sets][azure_virtual_machine_scale_sets].
+
+~> The Azure VMSS target plugin requires Azure fingerprinting, which became
+available in Nomad 1.0. If you are running a previous version of Nomad you will
+to set a [meta tag][client_meta_tag] in all of your nodes.
+
+### Agent Configuration Options
+
+To use the Azure VMSS target plugin, the agent configuration needs to be populated
+with the appropriate target block. Authentication to the Azure API can be supplied
+in a number of ways.
+
+
+#### Virtual Machine Identities
+
+When using [virtual machine identities][vm_identity] you will need to provide a
+user-managed identity with the `Contributor` role and set the `subscription_id`
+in the Autoscaler configuration file:
+
+```hcl
+target "azure-vmss" {
+  driver = "azure-vmss"
+  config = {
+    subscription_id   = "ee0886ab-5cc2-4583-a3f0-c4bfd044ee82"
+  }
+}
+```
+
+#### Configuration file
+
+Credentials should be injected into the configuration via a template rather
+than as environment variables. This ensures the credentials are passed only to
+the plugin, rather than being available for all plugins and the agent process.
+It is recommended, if possible to use the
+[Vault Azure Secrets engine][vault_azure_backend] for supplying access
+credentials to the plugin.
+
+```hcl
+target "azure-vmss" {
+  driver = "azure-vmss"
+  config = {
+    tenant_id         = "02e99e69-91b0-4d0a-bd61-bf5c08fbfbab"
+    client_id         = "34c276fa-2a74-4cf9-9fce-4e60642274cb"
+    secret_access_key = "ThisIrznTIS_~FAKEj4X93FTyAgilUC511"
+    subscription_id   = "ee0886ab-5cc2-4583-a3f0-c4bfd044ee82"
+  }
+}
+```
+
+- `tenant_id` `(string: "")` - The ID of the tenant to authenticate to.
+
+- `client_id` `(string: "")` - The app ID of the user-assigned identity in Azure
+  Active Directory.
+
+- `secret_access_key` `(string: "")` - The secret key ID used to authenticate
+  with the Azure API.
+
+- `subscription_id` `(string: "")` - The ID of the subscription to authenticate
+  to.
+
+#### Nomad ACL
+
+When using a Nomad cluster with ACLs enabled, the plugin will require an ACL token
+which provides the following permissions:
+
+```hcl
+node {
+  policy = "write"
+}
+```
+
+### Policy Configuration Options
+
+```hcl
+check "clients-azure-vmss" {
+  ...
+  target "azure-vmss" {
+    resource_group      = "prod"
+    vm_scale_set        = "hashistack-client-set"
+    node_class          = "hashistack"
+    node_drain_deadline = "5m"
+    node_purge          = "true"
+  }
+  ...
+```
+
+- `resource_group` `(string: <required>)` - The name of the [Azure resource group][azure_resource_group]
+  within which the virtual machine scale set resides.
+
+- `vm_scale_set` `(string: <required>)` - The name of the Azure virtual machine
+  scale set to interact with when performing scaling actions.
+
+- `node_class` `(string: <required>)` - The Nomad [client node class][nomad_node_class]
+  identifier used to group nodes into a pool of resource.
+
+- `node_drain_deadline` `(duration: "15m")` The Nomad [drain deadline][nomad_node_drain_deadline]
+  to use when performing node draining actions.
+
+- `node_purge` `(bool: "false")` A boolean flag to determine whether Nomad clients should
+  be [purged][nomad_node_purge] when performing scale in actions.
+
+### Client meta tag
+
+The Azure VMSS target plugin requires Nomad 1.0 for Azure fingerprinting.
+Alternatively, you can set a [client meta tag][client_meta_tag_docs] called
+`unique.platform.azure.name` with the value being the Azure VMSS node name.
+This can be retrieved programatically via the
+[Azure Instance Metadata service][azure_instance_metadata].
+
 [nomad_node_class]: /docs/configuration/client#node_class
 [nomad_node_drain_deadline]: /api-docs/nodes#deadline
 [nomad_scaling_stanza]: /docs/job-specification/scaling
@@ -229,3 +347,14 @@ check "hashistack-allocated-cpu" {
 [nomad_namespace_parameter]: /docs/job-specification/job#namespace
 [nomad_region_parameter]: /docs/job-specification/job#region
 [nomad_autoscaler_policy]: /docs/autoscaling/policy
+[azure_virtual_machine_scale_sets]: https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/overview
+[vault_azure_backend]: https://www.vaultproject.io/docs/secrets/azure
+[azure_resource_group]: https://docs.microsoft.com/en-us/azure/azure-resource-manager/management/manage-resource-groups-portal#what-is-a-resource-group
+[nomad_task_group_target]: /docs/autoscaling/plugins/target#nomad-task-group-target
+[aws_asg_target]: /docs/autoscaling/plugins/target#aws-autoscaling-group-target
+[azure_vmss_target]: /docs/autoscaling/plugins/target#azure-virtual-machine-scale-set-target
+[vm_identity]: https://docs.microsoft.com/en-us/azure/active-directory/managed-identities-azure-resources/
+[client_meta_tag_docs]: https://www.nomadproject.io/docs/configuration/client#meta
+[client_meta_tag]: /docs/autoscaling/plugins/target#client-meta-tag
+[azure_instance_metadata]: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/instance-metadata-service
+[das_task_target]: /docs/autoscaling/plugins/target#dynamic-application-sizing-nomad-task-target