Implement song Duration prop

src/Discord.Net.Core/Entities/Activities/SpotifyGame.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 
@@ -13,6 +14,7 @@ namespace Discord
         public string SyncId { get; internal set; }
         public string SessionId { get; internal set; }
         public string[] Artists { get; internal set; }
+        public TimeSpan? Duration { get; internal set; }
 
         public override string ToString() => Name;
         private string DebuggerDisplay => $"{Name} (Spotify)";

src/Discord.Net.WebSocket/Extensions/EntityExtensions.cs

@@ -8,6 +8,7 @@ namespace Discord.WebSocket
             if (model.SyncId.IsSpecified)
             {
                 var assets = model.Assets.GetValueOrDefault()?.ToEntity();
+                var timestamps = model.Timestamps.IsSpecified ? model.Timestamps.Value.ToEntity() : null;
                 return new SpotifyGame
                 {
                     Name = model.Name,
@@ -15,7 +16,8 @@ namespace Discord.WebSocket
                     SyncId = model.SyncId.Value,
                     TrackAlbum = assets?[1]?.Text,
                     TrackTitle = model.Details.GetValueOrDefault(),
-                    Artists = model.State.GetValueOrDefault()?.Split(';')
+                    Artists = model.State.GetValueOrDefault()?.Split(';'),
+                    Duration = timestamps?.End - timestamps?.Start
                 };
             }