Add IncomingPacketsResentCount clientstack statistics

This records how many packets were indicated to be resends by clients
Not 100% reliable since clients can lie about resends, but usually would indicate if clients are not receiving UDP acks at all or in a manner they consider timely.

OpenSim/Region/ClientStack/Linden/UDP/LLUDPServer.cs

@@ -132,6 +132,19 @@ namespace OpenSim.Region.ClientStack.LindenUDP
                     stat => stat.Value = m_udpServer.IncomingOrphanedPacketCount,
                     StatVerbosity.Info));
 
+            StatsManager.RegisterStat(
+                new Stat(
+                    "IncomingPacketsResentCount",
+                    "Number of inbound packets that clients indicate are resends.",
+                    "",
+                    "",
+                    "clientstack",
+                    scene.Name,
+                    StatType.Pull,
+                    MeasuresOfInterest.AverageChangeOverTime,
+                    stat => stat.Value = m_udpServer.IncomingPacketsResentCount,
+                    StatVerbosity.Debug));
+
             StatsManager.RegisterStat(
                 new Stat(
                     "OutgoingUDPSendsCount",
@@ -317,6 +330,11 @@ namespace OpenSim.Region.ClientStack.LindenUDP
         /// </summary>
         internal int PacketsSentCount { get; set; }
 
+        /// <summary>
+        /// Record how many incoming packets are indicated as resends by clients.
+        /// </summary>
+        internal int IncomingPacketsResentCount { get; set; }
+
         /// <summary>
         /// Record how many inbound packets could not be recognized as LLUDP packets.
         /// </summary>
@@ -1467,6 +1485,11 @@ namespace OpenSim.Region.ClientStack.LindenUDP
 
             #region Incoming Packet Accounting
 
+            // We're not going to worry about interlock yet since its not currently critical that this total count
+            // is 100% correct
+            if (packet.Header.Resent)
+                IncomingPacketsResentCount++;
+
             // Check the archive of received reliable packet IDs to see whether we already received this packet
             if (packet.Header.Reliable && !udpClient.PacketArchive.TryEnqueue(packet.Header.Sequence))
             {