add meta to fetch invoice arg

public/paths/billing/invoices/invoice.yml

@@ -9,6 +9,21 @@ get:
       required: true
       schema:
         type: string
+    - name: meta
+      in: query
+      required: false
+      description:
+        A comma separated list of meta values. Meta values will show up under a resource's
+        `meta` field. In the case of applying a meta to a collection of resources, each
+        resource will have it's own relevant meta data. In some rare cases, meta may not
+        apply to individual resources, and may appear in the root document. These will be
+        clearly labeled.
+      schema:
+        type: array
+        items:
+          type: string
+          enum:
+            - due
   summary: Fetch Billing Invoice
   description: Requires the `billing-invoices-view` capability.
   responses: