add instance metrics to server

AMNet.Server/DllMain.cs

@@ -1,3 +1,4 @@
+using System;
 using System.IO;
 using System.Runtime.InteropServices;
 using System.Text;
@@ -13,6 +14,10 @@ public static class DllMain
 {
     private const string WebAddress = "http://card.ppc.moe";
 
+    // instance metrics
+    internal static long LastPollTime = -1;
+    internal static long ServerStartedAt;
+
     static DllMain()
     {
         PInvoke.AllocConsole();
@@ -38,6 +43,7 @@ public static class DllMain
             cancellationRegistration?.Dispose();
         });
 
+        Volatile.Write(ref ServerStartedAt, Environment.TickCount64);
         App.RunAsync();
         return 0;
     }
@@ -45,6 +51,8 @@ public static class DllMain
     [UnmanagedCallersOnly(EntryPoint = "aime_io_nfc_poll")]
     public static int NfcPoll(byte unitNo)
     {
+        Volatile.Write(ref LastPollTime, Environment.TickCount64);
+
         if (!Config.EnableAimeTxt)
         {
             return 1;