fix(AIP-154): allow behavior annotations on request etag (#953)

Author: noahdietz

aip/general/0154.md

@@ -38,7 +38,8 @@ message Book {
 ```
 
 - The etag field **must** be a string, and **must** be named `etag`.
-- The etag field **should not** be given any behavior annotations
+- The etag field on the _resource_ **should not** be given any behavior
+  annotations.
 - The etag field **must** be provided by the server on output, and values
   **should** conform to [RFC 7232][].
 - If a user sends back an etag which matches the current etag value, the
@@ -78,10 +79,13 @@ message DeleteBookRequest {
   // The current etag of the book.
   // If an etag is provided and does not match the current etag of the book,
   // deletion will be blocked and an ABORTED error will be returned.
-  string etag = 2;
+  string etag = 2 [(google.api.field_behavior) = OPTIONAL];
 }
 ```
 
+On a request message, the `etag` field **should** be given a behavior annotation
+- either `REQUIRED` or `OPTIONAL`. See AIP-203 for more information.
+
 An `etag` field **may** also be used on custom methods, similar to the example
 above.