add aggregations to alerting condition

modules/monitoring/main.tf

@@ -60,16 +60,30 @@ resource "google_monitoring_alert_policy" "uptime_alert" {
     display_name = "http check failing for ${var.monitored_domain}${var.polling_path}"
 
     condition_threshold {
-      filter          = <<-EOT
+      filter     = <<-EOT
         metric.type="monitoring.googleapis.com/uptime_check/check_passed" AND
         metric.label.check_id="${google_monitoring_uptime_check_config.https[count.index].uptime_check_id}" AND 
         resource.type="uptime_url"
       EOT
-      duration        = "60s"
-      comparison      = "COMPARISON_GT"
+      duration   = "60s"
+      comparison = "COMPARISON_GT"
+
       threshold_value = "1"
+
       trigger { count = "1" }
-    }
 
+      # I don't fully understand this stuff, but leaving this empty doesn't
+      # work; although it used to (either the API or the terrfaform provider
+      # has changed). This config was arrived at my following
+      # https://cloud.google.com/monitoring/uptime-checks via the dashboard,
+      # and then examining the differences that terraform wants to apply. It
+      # seems to work OK.
+      aggregations {
+        alignment_period     = "120s"
+        group_by_fields      = ["resource.*"]
+        cross_series_reducer = "REDUCE_COUNT_FALSE"
+        per_series_aligner   = "ALIGN_NEXT_OLDER"
+      }
+    }
   }
 }