Implemented StarTime and Uptime

src/core/Akka.API.Tests/CoreAPISpec.ApproveCore.approved.txt

@@ -315,8 +315,10 @@ namespace Akka.Actor
         public abstract Akka.Actor.IScheduler Scheduler { get; }
         public abstract Akka.Serialization.Serialization Serialization { get; }
         public abstract Akka.Actor.Settings Settings { get; }
+        public System.TimeSpan StartTime { get; }
         [System.ObsoleteAttribute("Use WhenTerminated instead. This property will be removed in future versions")]
         public abstract System.Threading.Tasks.Task TerminationTask { get; }
+        public System.TimeSpan Uptime { get; }
         public abstract System.Threading.Tasks.Task WhenTerminated { get; }
         public abstract Akka.Actor.IActorRef ActorOf(Akka.Actor.Props props, string name = null);
         public abstract Akka.Actor.ActorSelection ActorSelection(Akka.Actor.ActorPath actorPath);

src/core/Akka/Actor/ActorSystem.cs

@@ -12,6 +12,7 @@
 using Akka.Configuration;
 using Akka.Dispatch;
 using Akka.Event;
+using Akka.Util;
 
 namespace Akka.Actor
 {
@@ -73,6 +74,16 @@ public abstract class ActorSystem : IActorRefFactory, IDisposable
         /// <summary>Gets the log</summary>
         public abstract ILoggingAdapter Log { get; }
 
+        /// <summary>
+        /// Start-up time since the epoch.
+        /// </summary>
+        public TimeSpan StartTime { get; } = DateTime.UtcNow - new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+
+        /// <summary>
+        /// Up-time of this actor system.
+        /// </summary>
+        public TimeSpan Uptime => MonotonicClock.ElapsedHighRes;
+
         /// <summary>
         ///     Creates a new ActorSystem with the specified name, and the specified Config
         /// </summary>