NodeGetVolumeStats feature does not work with Kubernetes

[tksm]

Thank you for implementing NodeGetVolumeStats in quobyte-csi v1.5.1.
I immediately tried this feature with Kubernetes v1.19, but I could not get kubelet_volume_stats_* metrics by kubelet.

According to NodeGetVolumeStats section in CSI Spec v1.3.0, it seems that quobyte-csi needs to return GET_VOLUME_STAT node capability. kubelet also checks this capability whether the driver support VolumeStats or not.

I confirmed that adding GET_VOLUME_STATS capability to NodeGetCapabilities() would fix this problem.

diff --git a/driver/node.go b/driver/node.go
index ad571ac..4737736 100644
--- a/driver/node.go
+++ b/driver/node.go
@@ -166,7 +166,7 @@ func (d *QuobyteDriver) NodeGetCapabilities(ctx context.Context, req *csi.NodeGe
                        {
                                Type: &csi.NodeServiceCapability_Rpc{
                                        Rpc: &csi.NodeServiceCapability_RPC{
-                                               Type: csi.NodeServiceCapability_RPC_UNKNOWN,
+                                               Type: csi.NodeServiceCapability_RPC_GET_VOLUME_STATS,
                                        },
                                },
                        },