[perf] Avoid boxing TagList in Activity.AddException

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/Activity.cs

@@ -1755,6 +1755,19 @@ public TagsLinkedList(IEnumerator<KeyValuePair<string, object?>> e)
                 }
             }
 
+            public TagsLinkedList(ReadOnlySpan<KeyValuePair<string, object?>> tags)
+            {
+                Debug.Assert(tags.Length > 0);
+
+                _last = _first = new DiagNode<KeyValuePair<string, object?>>(tags[0]);
+
+                for (int i = 1; i < tags.Length; i++)
+                {
+                    _last.Next = new DiagNode<KeyValuePair<string, object?>>(tags[i]);
+                    _last = _last.Next;
+                }
+            }
+
             public DiagNode<KeyValuePair<string, object?>>? First => _first;
 
             public TagsLinkedList(IEnumerable<KeyValuePair<string, object?>> list) => Add(list);

src/libraries/System.Diagnostics.DiagnosticSource/src/System/Diagnostics/ActivityEvent.cs

@@ -31,7 +31,19 @@ public ActivityEvent(string name) : this(name, DateTimeOffset.UtcNow, tags: null
         /// <param name="tags">Event Tags.</param>
         public ActivityEvent(string name, DateTimeOffset timestamp = default, ActivityTagsCollection? tags = null) : this(name, timestamp, tags, tags is null ? 0 : tags.Count) { }
 
-        internal ActivityEvent(string name, DateTimeOffset timestamp, ref TagList tags) : this(name, timestamp, tags, tags.Count) { }
+        internal ActivityEvent(string name, DateTimeOffset timestamp, ref TagList tags)
+        {
+            Name = name ?? string.Empty;
+            Timestamp = timestamp != default ? timestamp : DateTimeOffset.UtcNow;
+
+#if NET
+            // Read directly from the TagList's span so we avoid boxing the TagList (and its
+            // enumerator) as an IEnumerable<T> just to copy its tags into the linked list below.
+            _tags = tags.Count > 0 ? new Activity.TagsLinkedList(tags.Tags) : null;
+#else
+            _tags = tags.Count > 0 ? new Activity.TagsLinkedList((IEnumerable<KeyValuePair<string, object?>>)tags) : null;
+#endif
+        }
 
         private ActivityEvent(string name, DateTimeOffset timestamp, IEnumerable<KeyValuePair<string, object?>>? tags, int tagsCount)
         {