billingbudget: make threshold optional (GoogleCloudPlatform#6259)

mmv1/products/billingbudget/api.yaml

@@ -286,7 +286,6 @@ objects:
               use the `specified_amount` block.
       - !ruby/object:Api::Type::Array
         name: thresholdRules
-        required: true
         description: |
           Rules that trigger alerts (notifications of thresholds being
           crossed) when spend exceeds the specified percentages of the
@@ -336,6 +335,7 @@ objects:
               https://cloud.google.com/billing/docs/how-to/budgets#notification_format.
           - !ruby/object:Api::Type::Array
             name: monitoringNotificationChannels
+            max_size: 5
             at_least_one_of:
               - all_updates_rule.0.pubsub_topic
               - all_updates_rule.0.monitoring_notification_channels

mmv1/products/billingbudget/terraform.yaml

@@ -63,6 +63,15 @@ overrides: !ruby/object:Overrides::ResourceOverrides
           display_name: 'Example Billing Budget'
         test_env_vars:
           billing_acct: :BILLING_ACCT
+      - !ruby/object:Provider::Terraform::Examples
+        name: 'billing_budget_optional'
+        primary_resource_id: 'budget'
+        skip_docs: true
+        vars:
+          display_name: 'Example Billing Budget'
+          topic_name: "example-topic"
+        test_env_vars:
+          billing_acct: :BILLING_ACCT          
     properties:
       name: !ruby/object:Overrides::Terraform::PropertyOverride
         custom_flatten: 'templates/terraform/custom_flatten/name_from_self_link.erb'

mmv1/templates/terraform/examples/billing_budget_optional.tf.erb

@@ -0,0 +1,24 @@
+data "google_billing_account" "account" {
+  billing_account = "<%= ctx[:test_env_vars]['billing_acct'] -%>"
+}
+
+resource "google_billing_budget" "<%= ctx[:primary_resource_id] %>" {
+  billing_account = data.google_billing_account.account.id
+  display_name = "<%= ctx[:vars]['display_name'] %>"
+
+  amount {
+    specified_amount {
+      currency_code = "USD"
+      units = "100000"
+    }
+  }
+
+  all_updates_rule {
+    disable_default_iam_recipients = true
+    pubsub_topic = google_pubsub_topic.<%= ctx[:primary_resource_id] %>.id
+  }
+}
+
+resource "google_pubsub_topic" "<%= ctx[:primary_resource_id] %>" {
+  name = "<%= ctx[:vars]['topic_name'] %>"
+}