update CHANGELGO.md for #9321

CHANGELOG.md

@@ -9,6 +9,7 @@ IMPROVEMENTS:
 * dependencies: updating `msi` to API version `2018-11-30` [GH-10174]
 * Data Source: `azurerm_kubernetes_cluster` - support for the field `user_assigned_identity_id` within the `identity` block [GH-8737]
 * `azurerm_api_management` - support additional TLS ciphers within the `security` block [GH-9276]
+* `azurerm_api_management_api_diagnostic` - support the `sampling_percentage` property [GH-9321]
 * `azurerm_container_group` - support for updating `tags` [GH-10210]
 * `azurerm_kubernetes_cluster` - the field `type` within the `identity` block can now be set to `UserAssigned` [GH-8737]
 * `azurerm_kubernetes_cluster` - support for the field `new_pod_scale_up_delay` within the `auto_scaler_profile` block [GH-9291]