Root the System.Runtime EventSource (dotnet#108266)

Root the System.Runtime EventSource

The System.Runtime EventSource (RuntimeEventSource), was unintentionally being garbage collected because it wasn't rooted. This caused runtime EventCounters to no longer be available after GC occurred.

This was a regression from a recent change (dotnet#106014). That change accidentally converted the static field that was intended to the root the object into a property getter that returned a new instance each time it was called. This fix converts the property back to being initialized only once.

This will fix dotnet#107919 once it is backported.

Co-authored-by: Tarek Mahmoud Sayed <10833894+tarekgh@users.noreply.github.com>

src/libraries/System.Private.CoreLib/src/System/Diagnostics/Tracing/RuntimeEventSource.cs

@@ -23,7 +23,8 @@ public static class Keywords
             public const EventKeywords ProcessorCount = (EventKeywords)0x2;
         }
 
-        internal static RuntimeEventSource? Log => new RuntimeEventSource();
+        // this roots the singleton instance of the event source
+        internal static RuntimeEventSource Log { get; } = new RuntimeEventSource();
         private PollingCounter? _gcHeapSizeCounter;
         private IncrementingPollingCounter? _gen0GCCounter;
         private IncrementingPollingCounter? _gen1GCCounter;