Update package_version timer metadata string (#3761)

com.unity.ml-agents/Runtime/Academy.cs

         void InitializeEnvironment()
         {
             TimerStack.Instance.AddMetadata("communication_protocol_version", k_ApiVersion);
-            TimerStack.Instance.AddMetadata("package_version", k_PackageVersion);
+            TimerStack.Instance.AddMetadata("com.unity.ml-agents_version", k_PackageVersion);
 
             EnableAutomaticStepping();