build(terraform): Update Kinesis CloudWatch Alarm Lifecycle (#145)

* build(terraform): Update CloudWatch Alarm Lifecycle

* docs(terraform): Added Comments

* build(terraform): Ignore LastScalingEvent Tag

build/terraform/aws/kinesis_data_stream/main.tf

@@ -1,3 +1,14 @@
+locals {
+  # These are managed by the Autoscale application.
+  # https://github.com/brexhq/substation/blob/main/internal/aws/cloudwatch/cloudwatch.go
+  cw_alarm_ignore_changes = [
+    "datapoints_to_alarm",
+    "evaluation_periods",
+    "threshold",
+    "metric_query",
+  ]
+}
+
 resource "random_uuid" "id" {}
 
 resource "aws_kinesis_stream" "stream" {
@@ -10,7 +21,7 @@ resource "aws_kinesis_stream" "stream" {
   tags = var.tags
 
   lifecycle {
-    ignore_changes = [shard_count, tags]
+    ignore_changes = [shard_count, tags["LastScalingEvent"]]
   }
 }
 
@@ -92,7 +103,7 @@ resource "aws_cloudwatch_metric_alarm" "metric_alarm_downscale" {
   treat_missing_data  = "ignore"
 
   lifecycle {
-    ignore_changes = [metric_query, datapoints_to_alarm]
+    ignore_changes = local.cw_alarm_ignore_changes
   }
 
   metric_query {
@@ -177,7 +188,7 @@ resource "aws_cloudwatch_metric_alarm" "metric_alarm_upscale" {
   treat_missing_data  = "ignore"
 
   lifecycle {
-    ignore_changes = [metric_query, datapoints_to_alarm]
+    ignore_changes = local.cw_alarm_ignore_changes
   }
 
   metric_query {