feat(AIP-136): prohibit "async" in custom method RPC names

Author: jskeet

aip/general/0136.md

@@ -44,6 +44,11 @@ services. The bullets below apply in all three cases.
   - The name **must not** contain prepositions ("for", "with", etc.).
   - The verb in the name **should not** contain any of the standard method verbs ([Get][],
     [List][], [Create][], [Update][], [Delete][]).
+  - The name **must not** include the term `Async`. Instead, if the intention is
+    to differentiate between immediate and long-running RPCs, the suffix `LongRunning`
+    **may** be used for this purpose. For example, to create a long-running book creation
+    RPC (if the standard `CreateBook` method was designed before long-running aspects were
+    considered), a custom `CreateBookLongRunning` method could be introduced.
 - The HTTP method **must** be `GET` or `POST`:
   - `GET` **must** be used for methods retrieving data or resource state.
   - `POST` **must** be used if the method has side effects or mutates resources
@@ -159,8 +164,19 @@ without risk of runtime impact.
 [update]: ./0134.md
 [delete]: ./0135.md
 
+### RPC name
+
+The term "async" is commonly used in programming languages to indicate whether
+a specific method call is synchronous or asynchronous, including for making RPCs.
+That sync/async aspect is at a different abstraction level to whether the RPC
+itself is intended to start a long-running operation. Using "async" within the
+RPC name itself causes confusion, and can even cause issues for client libraries
+which generate both synchronous and asynchronous methods to call the RPC in some
+languages.
+
 ## Changelog
 
+- **2023-05-16:** Added prohibition of the term "async" within RPC names.
 - **2023-05-09:** Adding guidance for POST and GET, require parent instead of
   the resource singular.
 - **2023-03-02:** Explicitly discourage use of standard method verbs.