add terraform for low instance alert

operations/template/alert.tf

@@ -134,4 +134,45 @@ resource "azurerm_monitor_scheduled_query_rules_alert" "rs_sftp_log_errors_alert
       tags["zone"]
     ]
   }
+}
+
+resource "azurerm_monitor_metric_alert" "low_instance_count_alert" {
+  count               = local.non_pr_environment ? 1 : 0
+  name                = "cdc-rs-sftp-${var.environment}-azure-low-instance-count-alert"
+  resource_group_name = data.azurerm_resource_group.group.name
+  scopes              = [azurerm_monitor_autoscale_setting.sftp_autoscale.id]
+  description         = "The instance count in ${var.environment} is too low"
+  severity            = 2       // warning
+  frequency           = "PT1M"  // Checks every 1 minute
+  window_size         = "PT15M" // Every Check, looks back 15 minutes in history
+
+  criteria {
+    metric_namespace = "Microsoft.Insights/autoscalesettings"
+    metric_name      = "ObservedCapacity"
+    aggregation      = "Average"
+    operator         = "LessThanOrEqual"
+    threshold        = azurerm_monitor_autoscale_setting.sftp_autoscale.profile[0].capacity[0].default - 0.5
+  }
+
+  action {
+    action_group_id = data.azurerm_monitor_action_group.notify_slack_email[count.index].id
+  }
+
+  lifecycle {
+    # Ignore changes to tags because the CDC sets these automagically
+    ignore_changes = [
+      tags["business_steward"],
+      tags["center"],
+      tags["environment"],
+      tags["escid"],
+      tags["funding_source"],
+      tags["pii_data"],
+      tags["security_compliance"],
+      tags["security_steward"],
+      tags["support_group"],
+      tags["system"],
+      tags["technical_steward"],
+      tags["zone"]
+    ]
+  }
 }