private com.google.protobuf.SingleFieldBuilderV3<
            com.google.protobuf.FieldMask,
            com.google.protobuf.FieldMask.Builder,
            com.google.protobuf.FieldMaskOrBuilder>
        getReadMaskFieldBuilder() {
      if (readMaskBuilder_ == null) {
        readMaskBuilder_ =
            new com.google.protobuf.SingleFieldBuilderV3<
                com.google.protobuf.FieldMask,
                com.google.protobuf.FieldMask.Builder,
                com.google.protobuf.FieldMaskOrBuilder>(
                getReadMask(), getParentForChildren(), isClean());
        readMask_ = null;
      }
      return readMaskBuilder_;
    }
<pre>
Optional. Read mask is used for a more granular control over what the API
returns. If the mask is not present all fields will be returned except
[Queue.stats]. [Queue.stats] will be returned only if it was  explicitly
specified in the mask.
</pre>

<code>.google.protobuf.FieldMask read_mask = 5 [(.google.api.field_behavior) = OPTIONAL];
</code>
java-tasks/proto-google-cloud-tasks-v2beta2/src/main/java/com/google/cloud/tasks/v2beta2/ListQueuesRequest.java