Use UpDownCounter for grain and system target counts

UpDownCounter and Counter have the same implementation but they are interpreted differently by the .Net OTel SDK.
UpDownCounter supports reporting negative values and is suitable to keep track of the number of active grain per type.
The metric will be exported by the OTel SDK as a cumulative value instead of a delta for Counter.

src/Orleans.Core/Diagnostics/Metrics/GrainInstruments.cs

@@ -10,7 +10,7 @@ static GrainInstruments()
         GrainMetricsListener.Start();
     }
 
-    internal static Counter<int> GrainCounts = Instruments.Meter.CreateCounter<int>(InstrumentNames.GRAIN_COUNTS);
+    internal static UpDownCounter<int> GrainCounts = Instruments.Meter.CreateUpDownCounter<int>(InstrumentNames.GRAIN_COUNTS);
     internal static void IncrementGrainCounts(string grainTypeName)
     {
         GrainCounts.Add(1, new KeyValuePair<string, object>("type", grainTypeName));
@@ -20,7 +20,7 @@ internal static void DecrementGrainCounts(string grainTypeName)
         GrainCounts.Add(-1, new KeyValuePair<string, object>("type", grainTypeName));
     }
 
-    internal static Counter<int> SystemTargetCounts = Instruments.Meter.CreateCounter<int>(InstrumentNames.SYSTEM_TARGET_COUNTS);
+    internal static UpDownCounter<int> SystemTargetCounts = Instruments.Meter.CreateUpDownCounter<int>(InstrumentNames.SYSTEM_TARGET_COUNTS);
     internal static void IncrementSystemTargetCounts(string systemTargetTypeName)
     {
         SystemTargetCounts.Add(1, new KeyValuePair<string, object>("type", systemTargetTypeName));