chore(AIP-156): require singular and plural in resource (#1328)

Author: noahdietz

aip/general/0156.md

@@ -26,6 +26,8 @@ message Config {
   option (google.api.resource) = {
     type: "api.googleapis.com/Config"
     pattern: "users/{user}/config"
+    singular: "config"
+    plural: "configs"
   };
 
   // additional fields including name
@@ -55,6 +57,8 @@ rpc UpdateConfig(UpdateConfigRequest) returns (Config) {
   - Example: `users/1234/config`
 - Singleton resources are always singular.
   - Example: `users/1234/thing`
+- Singleton resource defintions **must** provide both the `singular` and
+  `plural` fields (see above example).
 - Singleton resources **may** parent other resources.
 - Singleton resources **must not** define the [`Create`][aip-133] or
   [`Delete`][aip-135] standard methods. The singleton is implicitly created or
@@ -108,8 +112,16 @@ Furthermore, presenting the Singleton resource as a pseudo-collection in such
 methods enables future expansion to a real collection, should a Singleton be
 found lacking.
 
+### Including `plural` definition
+
+While a Singleton is by definition singular, there are certain cases where
+a Singleton resource may appear in a plural form e.g., if the service supports
+Standard List (as defined here). As such, it is better to forward declare the
+plural form of the Singleton resource type than to not have it when needed.
+
 ## Changelog
 
+- **2024-04-15:** Singletons must specify `singular` and `plural` in resource.
 - **2023-08-10:** Add Standard `List` support.
 - **2023-07-26:** Clarified that read-only singletons should not have `Update`.
 - **2021-11-02:** Added an example message and state parent eligibility.