lnrpc+rpc: expose last ping message received by peer over ListPeers resp

lnrpc/lightning.proto

@@ -1561,6 +1561,11 @@ message Peer {
     zero, we have not observed any flaps for this peer.
     */
     int64 last_flap_ns = 14;
+
+    /*
+    The last ping payload the peer has sent to us.
+    */
+    bytes last_ping_payload = 15;
 }
 
 message TimestampedError {

lnrpc/lightning.swagger.json

@@ -5179,6 +5179,11 @@
           "type": "string",
           "format": "int64",
           "description": "The timestamp of the last flap we observed for this peer. If this value is\nzero, we have not observed any flaps for this peer."
+        },
+        "last_ping_payload": {
+          "type": "string",
+          "format": "byte",
+          "description": "The last ping payload the peer has sent to us."
         }
       }
     },

rpcserver.go

@@ -2682,16 +2682,17 @@ func (r *rpcServer) ListPeers(ctx context.Context,
 		)
 
 		rpcPeer := &lnrpc.Peer{
-			PubKey:    hex.EncodeToString(nodePub[:]),
-			Address:   serverPeer.Conn().RemoteAddr().String(),
-			Inbound:   serverPeer.Inbound(),
-			BytesRecv: serverPeer.BytesReceived(),
-			BytesSent: serverPeer.BytesSent(),
-			SatSent:   satSent,
-			SatRecv:   satRecv,
-			PingTime:  serverPeer.PingTime(),
-			SyncType:  lnrpcSyncType,
-			Features:  features,
+			PubKey:          hex.EncodeToString(nodePub[:]),
+			Address:         serverPeer.Conn().RemoteAddr().String(),
+			Inbound:         serverPeer.Inbound(),
+			BytesRecv:       serverPeer.BytesReceived(),
+			BytesSent:       serverPeer.BytesSent(),
+			SatSent:         satSent,
+			SatRecv:         satRecv,
+			PingTime:        serverPeer.PingTime(),
+			SyncType:        lnrpcSyncType,
+			Features:        features,
+			LastPingPayload: serverPeer.LastRemotePingPayload(),
 		}
 
 		var peerErrors []interface{}