feat: add RTT tracking and first-PING delay to NatsClient

src/NATS.Server/NatsClient.cs

@@ -73,6 +73,11 @@ public sealed class NatsClient : IDisposable
     private int _pingsOut;
     private long _lastIn;
 
+    // RTT tracking
+    private long _rttStartTicks;
+    private long _rtt;
+    public TimeSpan Rtt => new(Interlocked.Read(ref _rtt));
+
     public TlsConnectionState? TlsState { get; set; }
     public bool InfoAlreadySent { get; set; }
 
@@ -321,6 +326,14 @@ public sealed class NatsClient : IDisposable
 
             case CommandType.Pong:
                 Interlocked.Exchange(ref _pingsOut, 0);
+                var rttStart = Interlocked.Read(ref _rttStartTicks);
+                if (rttStart > 0)
+                {
+                    var elapsed = DateTime.UtcNow.Ticks - rttStart;
+                    if (elapsed <= 0) elapsed = 1; // min 1 tick for Windows granularity
+                    Interlocked.Exchange(ref _rtt, elapsed);
+                }
+                _flags.SetFlag(ClientFlags.FirstPongSent);
                 break;
 
             case CommandType.Sub:
@@ -611,6 +624,13 @@ public sealed class NatsClient : IDisposable
         {
             while (await timer.WaitForNextTickAsync(ct))
             {
+                // Delay first PING until client has responded with PONG or 2 seconds elapsed
+                if (!_flags.HasFlag(ClientFlags.FirstPongSent)
+                    && (DateTime.UtcNow - StartTime).TotalSeconds < 2)
+                {
+                    continue;
+                }
+
                 var elapsed = Environment.TickCount64 - Interlocked.Read(ref _lastIn);
                 if (elapsed < (long)_options.PingInterval.TotalMilliseconds)
                 {
@@ -629,6 +649,7 @@ public sealed class NatsClient : IDisposable
                 var currentPingsOut = Interlocked.Increment(ref _pingsOut);
                 _logger.LogDebug("Client {ClientId} sending PING ({PingsOut}/{MaxPingsOut})",
                     Id, currentPingsOut, _options.MaxPingsOut);
+                Interlocked.Exchange(ref _rttStartTicks, DateTime.UtcNow.Ticks);
                 WriteProtocol(NatsProtocol.PingBytes);
             }
         }