Add Name property to OtelMessage to store ChatMessage.AuthorName per OpenTelemetry semantic conventions (#6953)

* Initial plan

* Add Name property to OtelMessage and populate from ChatMessage.AuthorName

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Refactor: integrate AuthorName test into existing ExpectedInformationLogged_Async test

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: 2 people

src/Libraries/Microsoft.Extensions.AI/ChatCompletion/OpenTelemetryChatClient.cs

@@ -239,6 +239,7 @@ internal static string SerializeChatMessages(
                     message.Role == ChatRole.Tool ? "tool" :
                     message.Role == ChatRole.System || message.Role == new ChatRole("developer") ? "system" :
                     "user",
+                Name = message.AuthorName,
             };
 
             foreach (AIContent content in message.Contents)
@@ -595,6 +596,7 @@ private void AddOutputMessagesTags(ChatResponse response, Activity? activity)
     private sealed class OtelMessage
     {
         public string? Role { get; set; }
+        public string? Name { get; set; }
         public List<object> Parts { get; set; } = [];
         public string? FinishReason { get; set; }
     }

test/Libraries/Microsoft.Extensions.AI.Tests/ChatCompletion/OpenTelemetryChatClientTests.cs

@@ -105,10 +105,10 @@ async static IAsyncEnumerable<ChatResponseUpdate> CallbackAsync(
         List<ChatMessage> messages =
         [
             new(ChatRole.System, "You are a close friend."),
-            new(ChatRole.User, "Hey!"),
+            new(ChatRole.User, "Hey!") { AuthorName = "Alice" },
             new(ChatRole.Assistant, [new FunctionCallContent("12345", "GetPersonName")]),
             new(ChatRole.Tool, [new FunctionResultContent("12345", "John")]),
-            new(ChatRole.Assistant, "Hey John, what's up?"),
+            new(ChatRole.Assistant, "Hey John, what's up?") { AuthorName = "BotAssistant" },
             new(ChatRole.User, "What's the biggest animal?")
         ];
 
@@ -201,6 +201,7 @@ async static IAsyncEnumerable<ChatResponseUpdate> CallbackAsync(
                   },
                   {
                     "role": "user",
+                    "name": "Alice",
                     "parts": [
                       {
                         "type": "text",
@@ -230,6 +231,7 @@ async static IAsyncEnumerable<ChatResponseUpdate> CallbackAsync(
                   },
                   {
                     "role": "assistant",
+                    "name": "BotAssistant",
                     "parts": [
                       {
                         "type": "text",