fix: create the HzMemoryCache ActivitySource once instead of per call

HzMemoryCache/Diagnostics/HzActivities.cs

@@ -1,12 +1,13 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Diagnostics;
 
 namespace HzCache.Diagnostics
 {
     public static class HzActivities
     {
         public const string HzCacheActivitySourceName = "HzMemoryCache";
-        public static ActivitySource? Source => new(HzCacheActivitySourceName);
+        private static readonly ActivitySource source = new(HzCacheActivitySourceName);
+        public static ActivitySource Source => source;
 
         public static class Names
         {

UnitTests/DiagnosticsTests.cs

@@ -0,0 +1,49 @@
+using System.Diagnostics;
+using HzCache;
+using HzCache.Diagnostics;
+
+namespace UnitTests
+{
+    [TestClass]
+    public class DiagnosticsTests
+    {
+        [TestMethod]
+        public void SourceIsASingleInstance()
+        {
+            Assert.AreSame(HzActivities.Source, HzActivities.Source);
+        }
+
+        [TestMethod]
+        public async Task CacheOperationsDoNotCreateNewActivitySources()
+        {
+            // Every ActivitySource constructor registers the instance in the runtime's global list and
+            // invokes ShouldListenTo on all listeners, so any source with our name other than the one
+            // expected instance is a per-call construction. Touch Source first so the static instance
+            // exists before the listener is registered, regardless of test ordering.
+            var expected = HzActivities.Source;
+            var created = 0;
+            using var listener = new ActivityListener
+            {
+                ShouldListenTo = source =>
+                {
+                    if (source.Name == HzActivities.HzCacheActivitySourceName && !ReferenceEquals(source, expected))
+                        Interlocked.Increment(ref created);
+                    return false;
+                }
+            };
+            ActivitySource.AddActivityListener(listener);
+
+            using var cache = new HzMemoryCache(new HzCacheOptions { cleanupJobInterval = 20, notificationType = NotificationType.Sync });
+            for (var i = 0; i < 100; i++)
+            {
+                cache.GetOrSet($"key{i}", _ => new MockObject(i), TimeSpan.FromMinutes(1));
+                cache.Get<MockObject>($"key{i}");
+            }
+            cache.Remove("key0");
+            cache.RemoveByPattern("key1*");
+            await Task.Delay(200); // several cleanup-timer ticks
+
+            Assert.AreEqual(0, created);
+        }
+    }
+}