include grpc metadata in request requirement

Author: noahdietz

aip/client-libraries/4236.md

@@ -16,7 +16,8 @@ the per-API interface clients.
 
 If an API interface is annotated with `google.api.api_version`, client
 library generators **must** include either an HTTP query parameter `$apiVersion`
-or HTTP header `X-Goog-Api-Version`, but a request **must not** contain both.
+or HTTP header/gRPC metadata key `X-Goog-Api-Version`, but a request
+**must not** contain both.
 
 ### Generated client documentation