chore: make server streaming transformer safer (#2405)

Change-Id: I0fed7ea426a4b775a7fe5e661ed8e67f1faa53eb

Author: igorbernstein2

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/TransformingServerStreamingCallable.java

@@ -46,24 +46,24 @@ public void call(
 
     inner.call(
         innerReq,
-        new ResponseObserver<InnerRespT>() {
+        new SafeResponseObserver<InnerRespT>(outerObserver) {
           @Override
-          public void onStart(StreamController streamController) {
+          public void onStartImpl(StreamController streamController) {
             outerObserver.onStart(streamController);
           }
 
           @Override
-          public void onResponse(InnerRespT innerResp) {
+          public void onResponseImpl(InnerRespT innerResp) {
             outerObserver.onResponse(responseTransformer.apply(innerResp));
           }
 
           @Override
-          public void onError(Throwable throwable) {
+          public void onErrorImpl(Throwable throwable) {
             outerObserver.onError(throwable);
           }
 
           @Override
-          public void onComplete() {
+          public void onCompleteImpl() {
             outerObserver.onComplete();
           }
         },