add MonoPublishingTransformer and update ReactiveClient codegen to use publishOn

Summary: This follows the same pattern as the Flux transformer in D37835149

Reviewed By: prakashgayasen, sangongs

Differential Revision: D52818750

fbshipit-source-id: 9e03d6a598abbf7afc315c0fbdcdb7a8a5d1cdfd

Author: cnli87

third-party/thrift/src/thrift/compiler/generate/templates/java/ReactiveClient.mustache

@@ -213,7 +213,7 @@ public class {{service:javaCapitalName}}ReactiveClient {{#service:extends}}exten
 
             return _rpc
                 {{#function:oneway?}}.singleRequestNoResponse(_crp, rpcOptions).thenReturn(ResponseWrapper.create(null, java.util.Collections.emptyMap(), java.util.Collections.emptyMap())){{/function:oneway?}}{{!
-                }}{{^function:oneway?}}.singleRequestSingleResponse(_crp, rpcOptions).doOnNext(_p -> {if(_p.getException() != null) throw com.facebook.thrift.util.ExceptionUtil.propagate(_p);}){{/function:oneway?}};
+                }}{{^function:oneway?}}.singleRequestSingleResponse(_crp, rpcOptions).transform(com.facebook.thrift.util.MonoPublishingTransformer.getInstance()).doOnNext(_p -> {if(_p.getException() != null) throw com.facebook.thrift.util.ExceptionUtil.propagate(_p);}){{/function:oneway?}};
       }));
   }