Improve Akka.Cluster.Metrics collected values

src/contrib/cluster/Akka.Cluster.Metrics/Collectors/DefaultCollector.cs

@@ -55,12 +55,11 @@ public NodeMetrics Sample()
         {
             using (var process = Process.GetCurrentProcess())
             {
+                process.Refresh();
                 var metrics = new List<NodeMetrics.Types.Metric>()
                 {
                     // Memory
-                    // Forcing garbage collection to keep metrics more resilent to occasional allocations
-                    NodeMetrics.Types.Metric.Create(StandardMetrics.MemoryUsed, GC.GetTotalMemory(true)).Value,
-                    // VirtualMemorySize64 is not best idea here...
+                    NodeMetrics.Types.Metric.Create(StandardMetrics.MemoryUsed, process.WorkingSet64).Value,
                     NodeMetrics.Types.Metric.Create(StandardMetrics.MemoryAvailable, process.VirtualMemorySize64).Value,
                     // CPU Processors
                     NodeMetrics.Types.Metric.Create(StandardMetrics.Processors, Environment.ProcessorCount).Value,

src/contrib/cluster/Akka.Cluster.Metrics/StandardMetrics.cs

@@ -21,7 +21,7 @@ namespace Akka.Cluster.Metrics
     public static class StandardMetrics
     {
         /// <summary>
-        /// Total memory allocated to the currently running process (<see cref="GC.GetTotalMemory"/>)
+        /// Total memory allocated to the currently running process (<see cref="Process.WorkingSet64"/>)
         /// </summary>
         public const string MemoryUsed = "MemoryUsed";
         /// <summary>
@@ -84,7 +84,7 @@ public sealed class Memory
             /// </summary>
             public long Timestamp { get; }
             /// <summary>
-            /// The current process allocated memory (in bytes) (<see cref="GC.GetTotalMemory"/>)
+            /// The current process allocated memory (in bytes) (<see cref="Process.WorkingSet64"/>)
             /// </summary>
             public double Used { get; }
             /// <summary>

src/core/Akka/Util/Extensions/DateTimeExtensions.cs

@@ -14,12 +14,14 @@ namespace Akka.Util.Extensions
     /// </summary>
     public static class DateTimeExtensions
     {
+        private static readonly DateTime UnixOffset = new DateTime(1970, 1, 1);
+
         /// <summary>
         /// Converts given date and time to UNIX Timestamp - number of milliseconds elapsed since 1 Jan 1970
         /// </summary>
         public static long ToTimestamp(this DateTime dateTime)
         {
-            return (long)(dateTime - new DateTime(1970, 1, 1)).TotalMilliseconds;
+            return (long)(dateTime - UnixOffset).TotalMilliseconds;
         }
     }
 }