feat: add form submission health check metrics (#681)

Add a CloudWatch metric filter for successful and failed form submissions
through the client app.

aws/alarms/metrics.tf

@@ -0,0 +1,30 @@
+#
+# Metrics that will be used to monitor the behaviour and health of the app
+#
+locals {
+  healthcheck_metrics = [{
+    name           = "FormsClientSubmitSuccess"
+    pattern        = "Response submitted for Form ID"
+    log_group_name = var.ecs_cloudwatch_log_group_name
+    }, {
+    name           = "FormsClientSubmitFailed"
+    pattern        = "Attempted response submission for Form ID"
+    log_group_name = var.ecs_cloudwatch_log_group_name
+  }]
+}
+
+resource "aws_cloudwatch_log_metric_filter" "healthcheck" {
+  for_each = { for metric in local.healthcheck_metrics : metric.name => metric }
+
+  name           = each.value.name
+  pattern        = each.value.pattern
+  log_group_name = each.value.log_group_name
+
+  metric_transformation {
+    name          = each.value.name
+    namespace     = "forms"
+    value         = "1"
+    default_value = "0"
+    unit          = "Count"
+  }
+}