Splat.NLog - Reduce allocation when creating new NLogLogger (#1258)

* Splat.NLog - Reduce allocation when creating new NLogLogger

* Use generic Enum.GetValues available from NET5 and newer

src/Splat.NLog/NLogLogger.cs

@@ -14,6 +14,11 @@ namespace Splat.NLog;
 [DebuggerDisplay("Name={_inner.Name} Level={Level}")]
 public sealed class NLogLogger : IFullLogger, IDisposable
 {
+#if NET5_0_OR_GREATER
+    private static readonly LogLevel[] _allLogLevels = Enum.GetValues<LogLevel>();
+#else
+    private static readonly LogLevel[] _allLogLevels = Enum.GetValues(typeof(LogLevel)).Cast<LogLevel>().ToArray();
+#endif
     private readonly global::NLog.Logger _inner;
 
     /// <summary>
@@ -564,7 +569,7 @@ public LogLevel Level
     /// </remarks>
     private void SetLogLevel()
     {
-        foreach (LogLevel logLevel in Enum.GetValues(typeof(LogLevel)))
+        foreach (LogLevel logLevel in _allLogLevels)
         {
             if (_inner.IsEnabled(ResolveLogLevel(logLevel)))
             {