chore: ensure generation is included when a grpc ReadObject is retried (

#2741)

If a read is opened without a generation, we want to ensure we restart for the same generation. We have this behavior for json

This is mainly to avoid a race condition of reading an object (gen=1), that object being replaced (gen=2) then retry attempts to read it again we need to read gen=1 otherwise we will return corrupted data.

google-cloud-storage/src/main/java/com/google/cloud/storage/GapicUnbufferedReadableByteChannel.java

@@ -264,12 +264,15 @@ private void ensureStreamOpen() {
               alg,
               () -> {
                 ReadObjectObserver tmp = new ReadObjectObserver();
-                ReadObjectRequest request = req;
+                ReadObjectRequest.Builder builder = req.toBuilder();
                 long currentFetchOffset = fetchOffset.get();
-                if (request.getReadOffset() != currentFetchOffset) {
-                  request = req.toBuilder().setReadOffset(currentFetchOffset).build();
+                if (req.getReadOffset() != currentFetchOffset) {
+                  builder.setReadOffset(currentFetchOffset);
                 }
-                read.call(request, tmp);
+                if (metadata != null && req.getGeneration() == 0) {
+                  builder.setGeneration(metadata.getGeneration());
+                }
+                read.call(builder.build(), tmp);
                 ApiExceptions.callAndTranslateApiException(tmp.open);
                 return tmp;
               },

google-cloud-storage/src/test/java/com/google/cloud/storage/ITGapicUnbufferedReadableByteChannelTest.java

@@ -66,7 +66,8 @@ public final class ITGapicUnbufferedReadableByteChannelTest {
 
   private final ReadObjectRequest req1 =
       ReadObjectRequest.newBuilder().setObject(objectName).setReadOffset(0).build();
-  private final ReadObjectRequest req2 = req1.toBuilder().setReadOffset(20).build();
+  private final ReadObjectRequest req2 =
+      req1.toBuilder().setGeneration(3L).setReadOffset(20).build();
   private final ReadObjectResponse resp1 =
       ReadObjectResponse.newBuilder()
           .setMetadata(expectedResult)