feat: [billing] added currency field to billing account message (#5855)

* feat: added currency field to billing account message

PiperOrigin-RevId: 702355285

Source-Link: googleapis/googleapis@e5a3912

Source-Link: googleapis/googleapis-gen@82539f2
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packages/google-cloud-billing/README.md

@@ -44,7 +44,7 @@ Google APIs Client Libraries, in [Client Libraries Explained][explained].
 1.  [Select or create a Cloud Platform project][projects].
 1.  [Enable billing for your project][billing].
 1.  [Enable the Cloud Billing API][enable_api].
-1.  [Set up authentication with a service account][auth] so you can access the
+1.  [Set up authentication][auth] so you can access the
     API from your local workstation.
 
 ### Installing the client library
@@ -171,4 +171,4 @@ See [LICENSE](https://github.com/googleapis/google-cloud-node/blob/main/LICENSE)
 [projects]: https://console.cloud.google.com/project
 [billing]: https://support.google.com/cloud/answer/6293499#enable-billing
 [enable_api]: https://console.cloud.google.com/flows/enableapi?apiid=cloudbilling.googleapis.com
-[auth]: https://cloud.google.com/docs/authentication/getting-started
+[auth]: https://cloud.google.com/docs/authentication/external/set-up-adc-local

packages/google-cloud-billing/protos/google/cloud/billing/v1/cloud_billing.proto

@@ -285,6 +285,17 @@ message BillingAccount {
   //   - `billingAccounts/{billing_account_id}`, for example,
   //     `billingAccounts/012345-567890-ABCDEF`
   string parent = 6 [(google.api.field_behavior) = OUTPUT_ONLY];
+
+  // Optional. The currency in which the billing account is billed and charged,
+  // represented as an ISO 4217 code such as `USD`.
+  //
+  // Billing account currency is determined at the time of billing account
+  // creation and cannot be updated subsequently, so this field should not be
+  // set on update requests. In addition, a subaccount always matches the
+  // currency of its parent billing account, so this field should not be set on
+  // subaccount creation requests. Clients can read this field to determine the
+  // currency of an existing billing account.
+  string currency_code = 7 [(google.api.field_behavior) = OPTIONAL];
 }
 
 // Encapsulation of billing information for a Google Cloud Console project. A