feat(billing): attach attribution metadata to cloud subscriptions

app/javascript/dashboard/api/enterprise/account.js

@@ -1,5 +1,6 @@
 /* global axios */
 import ApiClient from '../ApiClient';
+import { getBillingAttribution } from '../../helper/billingAttribution';
 
 class EnterpriseAccountAPI extends ApiClient {
   constructor() {
@@ -11,7 +12,9 @@ class EnterpriseAccountAPI extends ApiClient {
   }
 
   subscription() {
-    return axios.post(`${this.url}subscription`);
+    return axios.post(`${this.url}subscription`, {
+      billing_attribution: getBillingAttribution(),
+    });
   }
 
   getLimits() {

app/javascript/dashboard/api/enterprise/specs/account.spec.js

@@ -42,7 +42,10 @@ describe('#enterpriseAccountAPI', () => {
     it('#subscription', () => {
       accountAPI.subscription();
       expect(axiosMock.post).toHaveBeenCalledWith(
-        '/enterprise/api/v1/subscription'
+        '/enterprise/api/v1/subscription',
+        {
+          billing_attribution: { visitor_id: undefined, session_id: undefined },
+        }
       );
     });