etcdserver: add ConfChangeAddLearnerNode to the list of config changes

To fix a panic that happens when trying to get ids of etcd members in
force new cluster mode, the issue happen if the cluster previously had
etcd learner nodes added to the cluster

Fixes etcd-io#12285

etcdserver/raft.go

@@ -614,10 +614,11 @@ func restartAsStandaloneNode(cfg ServerConfig, snapshot *raftpb.Snapshot) (types
 }
 
 // getIDs returns an ordered set of IDs included in the given snapshot and
-// the entries. The given snapshot/entries can contain two kinds of
+// the entries. The given snapshot/entries can contain three kinds of
 // ID-related entry:
 // - ConfChangeAddNode, in which case the contained ID will be added into the set.
 // - ConfChangeRemoveNode, in which case the contained ID will be removed from the set.
+// - ConfChangeAddLearnerNode, in which the contained ID will be added into the set.
 func getIDs(lg *zap.Logger, snap *raftpb.Snapshot, ents []raftpb.Entry) []uint64 {
 	ids := make(map[uint64]bool)
 	if snap != nil {
@@ -632,6 +633,8 @@ func getIDs(lg *zap.Logger, snap *raftpb.Snapshot, ents []raftpb.Entry) []uint64
 		var cc raftpb.ConfChange
 		pbutil.MustUnmarshal(&cc, e.Data)
 		switch cc.Type {
+		case raftpb.ConfChangeAddLearnerNode:
+			ids[cc.NodeID] = true
 		case raftpb.ConfChangeAddNode:
 			ids[cc.NodeID] = true
 		case raftpb.ConfChangeRemoveNode: