feat: copy labels from Disk to Snapshot

It can be very useful to be able to track your snapshots using the same labels are you have on your original disks, this PR copies over all of the labels of the disk and adds a new permission requirement of `compute.snapshots.setLabels` to allow for that.

README.md

@@ -117,6 +117,7 @@ These permissions are required by Velero to manage snapshot resources in the GCP
         compute.snapshots.create
         compute.snapshots.useReadOnly
         compute.snapshots.delete
+        compute.snapshots.setLabels
         compute.zones.get
         storage.objects.create
         storage.objects.delete

velero-plugin-for-gcp/volume_snapshotter.go

@@ -210,6 +210,7 @@ func (b *VolumeSnapshotter) CreateVolumeFromSnapshot(snapshotID, volumeType, vol
 		SourceSnapshot: res.SelfLink,
 		Type:           volumeType,
 		Description:    res.Description,
+		Labels:         res.Labels,
 	}
 
 	if isMultiZone(volumeAZ) {
@@ -318,6 +319,7 @@ func (b *VolumeSnapshotter) createSnapshot(snapshotName, volumeID, volumeAZ stri
 		Description:  getSnapshotTags(tags, disk.Description, b.log),
 		SourceDisk:   disk.SelfLink,
 		SnapshotType: b.snapshotType,
+		Labels:       disk.Labels,
 	}
 
 	if b.snapshotLocation != "" {
@@ -343,6 +345,7 @@ func (b *VolumeSnapshotter) createRegionSnapshot(snapshotName, volumeID, volumeR
 		Description:  getSnapshotTags(tags, disk.Description, b.log),
 		SourceDisk:   disk.SelfLink,
 		SnapshotType: b.snapshotType,
+		Labels:       disk.Labels,
 	}
 
 	if b.snapshotLocation != "" {