chore(storage): implement gRPC ListObjects read_mask support (#6808)

This adds `Query`-to-`FieldMask` conversion for `Object` attributes and incorporates that into `ListObjects` for gRPC.

This also carves out the `ListObjects`-specific tests from `TestIntegration_Objects` and enables those tests for multi-transport testing.

storage/grpc_client.go

@@ -380,14 +380,14 @@ func (c *grpcStorageClient) ListObjects(ctx context.Context, bucket string, q *Q
 		it.query = *q
 	}
 	req := &storagepb.ListObjectsRequest{
-		Parent:             bucketResourceName(globalProjectAlias, bucket),
-		Prefix:             it.query.Prefix,
-		Delimiter:          it.query.Delimiter,
-		Versions:           it.query.Versions,
-		LexicographicStart: it.query.StartOffset,
-		LexicographicEnd:   it.query.EndOffset,
-		// TODO(noahietz): Convert a projection to a FieldMask.
-		// ReadMask: q.Projection,
+		Parent:                   bucketResourceName(globalProjectAlias, bucket),
+		Prefix:                   it.query.Prefix,
+		Delimiter:                it.query.Delimiter,
+		Versions:                 it.query.Versions,
+		LexicographicStart:       it.query.StartOffset,
+		LexicographicEnd:         it.query.EndOffset,
+		IncludeTrailingDelimiter: it.query.IncludeTrailingDelimiter,
+		ReadMask:                 q.toFieldMask(), // a nil Query still results in a "*" FieldMask
 	}
 	if s.userProject != "" {
 		ctx = setUserProjectMetadata(ctx, s.userProject)
@@ -411,6 +411,12 @@ func (c *grpcStorageClient) ListObjects(ctx context.Context, bucket string, q *Q
 			it.items = append(it.items, b)
 		}
 
+		// Response is always non-nil after a successful request.
+		res := gitr.Response.(*storagepb.ListObjectsResponse)
+		for _, prefix := range res.GetPrefixes() {
+			it.items = append(it.items, &ObjectAttrs{Prefix: prefix})
+		}
+
 		return token, nil
 	}
 	it.pageInfo, it.nextFunc = iterator.NewPageInfo(
@@ -449,6 +455,8 @@ func (c *grpcStorageClient) GetObject(ctx context.Context, bucket, object string
 	req := &storagepb.GetObjectRequest{
 		Bucket: bucketResourceName(globalProjectAlias, bucket),
 		Object: object,
+		// ProjectionFull by default.
+		ReadMask: &fieldmaskpb.FieldMask{Paths: []string{"*"}},
 	}
 	if err := applyCondsProto("grpcStorageClient.GetObject", gen, conds, req); err != nil {
 		return nil, err

storage/http_client.go

@@ -344,8 +344,8 @@ func (c *httpStorageClient) ListObjects(ctx context.Context, bucket string, q *Q
 		req.EndOffset(it.query.EndOffset)
 		req.Versions(it.query.Versions)
 		req.IncludeTrailingDelimiter(it.query.IncludeTrailingDelimiter)
-		if len(it.query.fieldSelection) > 0 {
-			req.Fields("nextPageToken", googleapi.Field(it.query.fieldSelection))
+		if selection := it.query.toFieldSelection(); selection != "" {
+			req.Fields("nextPageToken", googleapi.Field(selection))
 		}
 		req.PageToken(pageToken)
 		if s.userProject != "" {