fix: add per-service apiVersion to gapic_metadata (#1239)

* fix: add per-service apiVersion to gapic_metadata

* use unless instead of if

* use unless instead of if for real this time

* go back to variable and conditional add

Author: noahdietz

gapic-generator/lib/gapic/presenters/service_presenter.rb

@@ -526,7 +526,7 @@ def grpc_full_name
       #
       # @return [Hash]
       def drift_manifest
-        {
+        h = {
           clients: {
             grpc: {
               libraryClient: client_name_full,
@@ -538,6 +538,8 @@ def drift_manifest
             }
           }
         }
+        h[:apiVersion] = @service.api_version unless @service.api_version.empty?
+        h
       end
 
       ##

shared/output/gapic/templates/showcase/gapic_metadata.json

@@ -126,7 +126,8 @@
             }
           }
         }
-      }
+      },
+      "apiVersion": "v1_20240408"
     },
     "Identity": {
       "clients": {